U N D E R S T A N D I N G C O L O U R :
There's enough info out there for understanding the colour wheel, and complementaries colours and all that jazz, so I won't be breaking down colour in this aspect. Instead this is where I will help you understand blacks and shadows, as well as how to get realistic flesh tones, and how colours work differently for various oil painting techniques, and how to see colour properly when you're struggling to mix the correct tone.
Some general info to note about colour before we get into shadows and flesh tones involve understanding that colour is a tool to control the viewer and control how the subject is seen. Utilizing colour correctly can give you control over how your work is seen. You can control where the viewer looks. Manipulate the viewer emotionally. Create tension, and narratives. It's so much more than just "colour".
The starting point to colour is understanding "warms" (red/orange/yellow) and "cools" (blue/green/purple) and how these aren't just a temperature but how they translate to distance.
As things go off into the distance and recede, they become cooler in tone (imagine standing in a field looking out to the mountains way off in the distance, things slowly begin to decrease in saturation and get cooler/ bluish in tone.
Where as the objects closer to you the viewer are more saturated and "warmer".
If cool recedes and warm pushes things closer, a way to create depth and distance in the painting is to use this to your advantage. Blue and desaturated backgrounds push the subject closer to the front of the painting and viewer, while a red background will envelop the subject. Think of what you want your piece to convey in that context and utilize colour appropriately to speak this through paint.
Our eyes are drawn to areas of a canvas that are most vibrant/ saturated, as well as the areas where there is the strongest point of discrepancy. If colours and values blend seamlessly, our eyes flow over the canvas, but if you want to control the viewer to pull their attention to one specific point, using colours that are jagged against one another, or using bright colours are one method, but one of the strongest subtle ways to do this is with the contrast between light and shadow- more specifically its strongest point of discrepancy- so black and white. Where ever you see a bright white highlight right up against the deepest black, we cant help but be drawn to it. (Think a bright white specular highlight on a black pupil). This is extremely important to note in painting, b/c it allows you to make all other blacks not as dark, and all other highlights not as bright, so that the starkest contrast is right where we want to pull attention.
It's important to know that you do not have to adhere to how you see a subject in life. You can flex your paint mastery by controlling specific elements subtly to manipulate the viewer without them knowing how/why. What makes your painting better than just photographing or drawing a piece. If you're going to go through the intense work to learn oil painting, you might as well utilize all the aspects which allow you to create a superior piece through this medium. That starts with understanding the mastery of colour.
It's important to note that when mixing oil colours the relationship between them is different when working wet on wet, vs opaque wet on dry, vs Transparent wet on opaque dry, vs transparent on transparent, vs dry brush on dry transparent, vs dry brush on dry opaque. You can take the same two colours and apply them in these different ways and end up with a different final tone each time.
BLACKS & SHADOWS:
So the first thing you'll want to understand is that "black" doesn't really exist. People tend to see black as the end-all-be-all darkest point of the colour spectrum. However black isn't one "colour". There are several options for black oil pant (lamp, ivory, mars, chromatic) and though they all have different tones and properties you would never use these to paint "black" in your oil painting. Instead these black oils are best used for mixing and tinting. The biggest mistake I see new painters making is using black as a colour on their palette to put as-is in the painting. DO NOT DO THIS! This is a surefire way to have your painting look flat, lack depth, and look muddy. Only use these pre-made ready-to-go blacks for mixing and tinting other colours!
Instead you need to break down black by taking a much closer examination of what it is, where it is, and what its made up of. Best to think of it as black being just a really dark version of an actual colour, or combo of colours. Most pre-made blacks are blue-black. You can see this the moment you mix it with other colours and it starts acting as though you've mixed in blue. You can also see this by thinning out the black with a fluid medium and see as it becomes more transparent, that it's really just dark blue. But not all black or shadows areas are blue in tone. Sometimes you'll have a red-black, or a brown-black, or green-black, or purple-black. A painting is only as strong as its weakest parts, and if all your focus is in the lighter areas, and you disregard the shadows, it will never be as strong as it can be. DO NOT SKIMP ON SHADOWS. Give the darker areas of your panting the same care and attention. If your shadows do not have depth, and look flat, you will lose all dimensionality and believability to the piece.This is why you will need to mix your own blacks.
When I need to use "black" in a painting, I always mix my own custom to the area that needs it from a combination of these colours:
-Alizirin Crimson
-Paynes Grey
-Raw Umber
-Asphaultum
-Phthalo green
-Tranparent Earth Red (for shadows transitioning in figurative and portrait work)
-Transparent Earth Orange (for shadows transitioning in figurative and portrait work)
-Tranparent Earth Yellow (for shadows transitioning in figurative and portrait work)
-Sometimes I'll add one of the pre-mixed blacks if I need extra depth (usually Mars for a warmer black, Ivory for a cooler black, or Chromatic or Spinnel for a neutral black) But if I use a pre-made black I have to mix it with at least two other of the above tones.
My most common black mix is: Alizarin, Paynes, and Raw Umber.
I adjust this combo to have more Alizarin if I need it warmer, more Paynes if I need it cooler, or Raw if I need it more neutral.
On your palette all of your black mixes will look the same. It will be difficult to see if its warm or cool or neutral enough, so it helps to get in the habit of always mixing them in the same location and remembering what blob is what. You can also mark a label on your palette. Even still, while mixing it can be hard to tell if you've got the tones where it needs to be, so just to make sure, before you lay it down, take a bit of galykd or liquin and thin a smidge down until its transparent enough that you can see. Or wipe the palette knife you used to mix it on a paper towel and as you drag it across and it thins out you can see its undertone.
It may take a bit to begin to see what black you'll need for your painting. But you can begin to train yourself to see what tones of black you'll need by breaking down the colours around it. Ask yourself a few quick questions to guide you:
-What tones is it blending into?
-Is it a shadow or a black object?
-Is it further in the background or closer to the forefront?
-Is it a glaze or opaque application
-Do I plan to glaze over it?
So if you're blending a black out, you'll want t make sure its going to blend appropriately in tone to the surrounding colours. Think of how they will blend together if its wet meeting wet, vs wet meeting dry or weather you're glazing. These all affect how it looks and blends. Don't use a cool black mix to something blending to an orangey tone- you'll get a weird muddy and flat transition. The number one reason people fail at creating believable portraits, is their transition from shadow to lit areas make the flesh tone muddy and flat b/c they mix black from a tube which is cool, into those warmer flesh tones. When instead they should be mixing some alizarin and maybe even one of the Transparent Earth tones into their black/shadow mix. The transition from a black shadow to flesh tones has so many colours! You need to take the time to paint this transition with care to the tones you see.
Whether it's a shadow or a black object you're painting will affect how you paint it and the colours you use. Every physical object (including people) reflect the light and colour of the objects they are surrounded by in their environment differently. Pay close attention to the relationship between the subject and its surrounding to see how blacks and shadows are affected by this.
As things go off into the distance and recede, they become cooler in tone (imagine standing in a field looking out to the mountains way off in the distance, things slowly begin to decrease in saturation and get cooler/ bluish in tone.
Where as the objects closer to you the viewer are more saturated and "warmer".
If cool recedes and warm pushes things closer, you need to consider this in your shadows, are you trying to push the shadow or object in question back, or are you trying to bring it closer to the forefront. Use cooler tones in your blacks to push things away, and warmer tones to keep them close.
Last thing to keep in mind when mixing the blacks for the area is if you plan to glaze over them in upcoming layers. If so, what colours will you be glazing. This will affect the tint of the over all finished black, so work that into the equation.
A quick note in BLACK AND WHITE:
If you're going to paint a piece all in black and white and use the standard "ivory black" and "titanium white", you will get a blue-ish monochrome painting since they both have cool tints. If you're looking for a neutral grey from "black and white" you're going to want to use Vandyke Brown instead. Mix a line of values out from black and notice it gets blue. Mix a value line out from VanDyke Brown and notice it gets neutral grey.
FLESH TONES:
There are pre-mixed "flesh tone" paints you can get, and though they can serve a purpose, you will never use them as your flesh tone colour when painting. This is b/c there is no one flesh tone. Not even on one person. Skin tones shift massively even within one person on one small part of their body.... take a look at your hand right now... see how many subtle tonal shifts there are?
When mixing flesh tones it's best to mix custom from a set of primary colours.
Zorn was able to create beautiful flesh tones using only 4 paint colours:
-Cadmium Red
-Yellow Ochre
-Titanium White
-Ivory Black
The white and black didn't just act as value adjusters, they acted as the blue. Mix the two together and you get blue since they are both cool tones. Once you have red, yellow, and blue, you can mix any colour.
But many more paint options are available these days and it can save you some trouble to have extra colours at your disposal rather than having to mix a custom colour just to use a tiny bit to mix in another colour. However having too many options can create incoherency between the tones and your subject can start looking bruised or patchy.
A painters flesh tone palette becomes super personal. I can tell you what mine is, but you may find you'd like to sub out colours for others, bring new colours in, or remove some all together. I highly encourage exploring what palette best works for you. It may take time to develop your signature platte. Don't get frustrated if it doesn't click right away.
For my flesh tone platte I use:
-Titanium White
-Titanium Buff
-Naples Yellow
-Transparent Earth Red, Orange, and Yellow. (mostly Transp. Earth Red and Transp Earth Orange.)
-Alizarin
-Gamblins Flesh tint
-Paynes
-Raw umber
-Burnt Unmber
-Asphaultum
-Chromatic black
-Mars Black
-Terre Vert (about half the time)
-Cadmium red (rarely makes it's way onto my palette but since I still use it, adding it in)
-Cadmium Orange (rarely makes it's way onto my palette but since I still use it, adding it in)
-Indigo (rarely makes it's way onto my palette but since I still use it, adding it in)
Looking at the flesh tone in question- is the skin a little more yellow? is it a little more olive? Is it a bit red? Pink? Is it so pale that where it becomes almost translucent looking, do you notice blue tints?
Like the black and shadow breakdown above, this applies to darker skin tones too. Darker skin isn't just "shades of brown". Like The shadows, it can be more difficult to see the tonal shifts in darker skin tones, or to pick out your primary tints, so you may need to take a closer look. There are areas more green, more red, more purple. It's easy for us to see "skin tone" instead of colours. So people tend to break it into either "peach" or "brown" and so they begin to mix these colours and use them for skin. Colour is so much more complex than this- especially in skin tones.
The most helpful advice I can offer is when you are painting a specific area and if it isn't reading well, look at it and think "can it use more yellow?" if you think 'no- b/c then it will look too jaundice', then ask "does it need more red?" and if you think "no that will make it too saturated or sun burned looking, then ask "does it need more blue?" it's a question you never think to ask b/c you don't think "blue" when you think skin tones- but you need it to make them. Now you may look at it and think "well no, it doesn't really need blue exactly, but it definitely doesn't need red or yellow... well then perhaps you need more of blue mixed with another primary- say for example blue+yellow to create green... ask yourself "does it need more green? this may be what you need for more olive tonal corrections. If thats not it, then ask blue+red for purple... This may be what you need for areas that aren't getting as much light and are partially transitioning to shadowed areas or just aren't getting as much light. Or if you painted it too yellow to start, you may just need purple tones to neutralize it (think complimentary colours).
So when you start, you'll want to sort of squint and look at the areas not in the most light, not in the shadows, and find your your base colour. This will be the foundation you mix for all the other tonal shifts to riff off of. So you'll want to take extra care when mixing at this stage. Constantly ask "does it need more red? Blue? Yellow? Mix 3 or four of these mid base tones just slightly different from the rest.
Then do the same for your shadow areas, and your lighter areas.
Your palette should then have 3 groupings of colours - your darks, mid tones, and light tones. and in each one, 3-4 swatches respectively. You may mix more, just ideally no less than 3. You will find if you mix too many, they don't get used, b/c as you blend, they mix in ways you need to create specific colours for that are now different from your reference. This is where layers of glazing come into play.
Another highly important thing to note with flesh tones is people look at details and confuse it as a single colour instead of seeing the layers of colours that come together to create it. Adding details will change the overall tone of that area. This is the second biggest mistake I see people make.
an example is eyebrows. You need to paint the flesh tone as if the person has no eyebrows. Then think the hairs create shadows on the flesh.. what colour are those shadows? layer them over the flesh tone, (not as one big swipe... but jagged as they are in life), then layer the actual brow hairs, then add highlights.
Every step of the way you need to imagine your layers as they are in life. The way each aspects interact with the aspect below and above it and how it all comes together in harmony.
The struggle with painting figures and portraits is they expect it to look like the finished face from the first layer. You need to have patience. It's going to look ass ugly until it begins to look good. It will look "off" until the final details go in, that alter the way we are seeing the tones and values. So as you're working you need to constantly be thinking of what is coming next and will that lighten it, or darken it? will that layer change this layer's feel? tone?
You need to be mindful of the process and TRUST THE PROCESS. You need to be okay with it looking like shit for a while. You need to not want to rush it and get certain colours or values or details in to quell that ache before those colours, values or details are ready to go in! Being impatient and adding it in a session to make you feel better looking at it for that session, will only hurt you in the end. This isn't drawing. This isn't acrylic. This isn't tattooing. YOU NEED PATIENCE. It fucking sucks, but turn the painting backwards if you have to not look at it when you're not working so it's ugly middle stages cant haunt you. Trust me the rewards of waiting to put everything in at the right time, is worth the emotional turmoil when you see it finally all come together.