Three colour primary palette mixing theory.
An example from www.spooks-art.com
Choose three primary colours.
In reality, no manufacturer can make perfect primary colours in a tube. If they could, then any two when mixed should make black. In practice, we need to understand the contribution of each commercial primary colour to the possible range of colours.
While you may be aware of a six-colour palette which overcomes some of the limitations of paint manufacture, this article concentrates on the simpler three colour palette.
Most beginners ask, "What paints should I buy?"
The answer they expect is often a long list of colours making up some kind of coveted secret formula, right down to a special arrangement on the palette.
In practice, you can get away with three colours near the primary ideal, and white. That's it.
Here is one recommendation:
Cadmium Red, French Ultramarine, Cadmium Yellow, Titanium White.
Please note: some paints and pigments vary in opacity as well as hue.
To make black:
Start with 4 parts blue, 2 parts blue, 1 part yellow.
Then, because different manufacturers have various formulae and provide differing strengths, make adjustments in the ratio until you get as deep a black as possible. This 'dark' is going to be more harmonious to your painting than introducing a foreign pre-manufactured black.
If your black looks green, add a touch of red.
If your black looks red, add a touch of blue and yellow.
If your black looks blue, add a touch of red and yellow.
If your black looks orange, add a touch of blue.
If your black looks yellow, add blue and red.
If your black looks purple, add yellow.
The above guidelines really embodies almost all that you need to know about mixing colours although you won't be able to appreciate this properly until understanding the theory first. Although it looks like a list of rules that are hard to remember, you don't need to remember them at all. There is a simpler method of mentally computing these rules. Read on and watch the video and I will show you.
A video on colour mixing
A special note about highlights.
In the video I said that I think of yellow as the sun, and this makes it easy to mentally place it up top. Also, to make a highlight, add yellow. That's all great, but if your light source is cool, then you will have to add blue to create a highlight. So that's the more general rule. To lighten a local colour, mix in something to move it nearer the colour temperature of the light source.
This then makes it very simple to bring in another rule... To make a shadow, mix in some of the colour that moves it towards the COMPLIMENT of the light source. In many cases, this is purple because a sun-light subject is often warm.
But if you are dealing with reflected-light off cold walls and other situations that alter the colour temperature of the sunlight, then you will need to make that assessment and determine just where on the colour wheel is the actual light that illuminates the subject. Just because the sun is warm, does not always mean that the illumination of the subject is also warm.
To re-state part of this rule in a more familiar form: "Warm light casts cool shadows, and cool light casts warm shadows."
... but you really do need to consider not the light source and its colour temperature, but the temperature of the light that finally illuminates the subject.
Just to drive this home, consider a lemon, next to a big red apple. Light coming from the sun from the top left. Consider the lemon to the left of the apple. The apple will reflect red light on to the lemon, therefore the highlight of the yellow lemon will be reddish, and the shadow cast from this reflected light to the left of the lemon will be cool.
Of course you also have to factor in the strong warm sunlight coming in from the top left, and how it would affect the shadow from the apple's reflected light - but that will depend on what else is in the picture.
But how do I mix brown?
Brown is a darkened orange. Make an orange, then add a blue. I say "an" orange and "a" blue because although we are working with just three primary colours, you can get a lot of shades of orange. There will be a blue that is opposite that orange. If your orange tends to yellow, then its shadow (brown) is created by adding a purply-blue. If your orange is tending towards red, then you will adding a greenish blue.
Incidentally, a very credible black can be made with a mix of a strong commercially mixed brown and strong blue.
A note about green.
When I was young, I mixed hundreds of blobs of paint together and charted them all. This gave me an intuitive feel for colour mixing. I noticed that green was most sensitive to colour mixing. There seemed to be so many possible greens. This is partly because the available red and yellows commercially available had different characteristics, and also because the human eye is most sensitive to green light. You might find that a particular green seems to be impossible to find using your three colour system. In this case, introduce a 'warm green' and a 'cool green' from a commercial mix. Then try again. You now have a four-colour mix. You can probably work out that the logical extension of this idea is a six-colour palette.
Over time, you are likely to find some favourite colours. There are no particular rules that must be adhered to, but to understand some of the physics involved in colour will improve your efficiency and results.
When I was doing these experiments, it was a surprise to find that black and yellow made green. Later in life, I found that this was because commercial mixes of black exploit the fact that blue/purple are low values and therefore, the construction of a black pigment will often contain a lot of blue. Adding yellow tends to bring this out.
Student quality paints are expensive!
This is because they have less pigment. Therefore, you will not be able to create a very good black, and you will also not be able to mix in much white before it becomes flat. You end up using much more paint than necessary, and it works out more expensive.
Artist-quality pigments can be very strong. In some cases, a pin-prick sized touch is all you need to transform a hue. So you use much less artist-quality paint than student quality paint, and the range of vibrancy that you can get in your painting is extended because there is so much more scope to move from the primary mix into the pastels. At some point, adding white washes out your colours, and therefore if you start off with a much stronger pigment, you get a wider range of vibrant colours as you add white before it starts to go bland.
There is more to learn...
Mixing colour is a complex subject. I've hopefully managed to give you a few tips. However, each colour also has what's called a value. This is the shade of grey that the colour moves to when you convert to black and white. Yellow has a high value, and its opposite (purple) has a low value. Black is the extreme case. White is the opposite extreme case of black.
It's sometimes hard to compare values when colour is involved, but there is a trick. Squint.
When you squint, it is like viewing in dim light which tends to remove our perception of colour. Luckily, this trick still lets us compare the amount of light reflected compared to some reference, or between objects. So by squinting, you will be able to judge how bright or dim say, your orange should be if you want a corresponding green that has the same value. When you squint at the two, they should look the same shade.
To control value is to understand how to create 3D effects from a 2D painting. This is why it is instructive to learn to draw in detail in black and white before you rush off and try to create abstract and impressionist work.