COLOUR ( Part 1)
It's true that individual colours have a bias towards cool or warm for colour mixing. If you mix two warms together, you'll get a warm secondary colour and, if you mix two cools together you'll get a cool secondary.
For example, mixing cadmium yellow and cadmium red light creates a warm orange. If you mix lemon yellow with alizarin crimson, you get a cooler, more gray orange. Mixing secondary colours is not only about the proportions in which you mix two primary colours, but also knowing what different reds, yellows, and blues produce.
As a group, reds and yellows are considered warm colours and blue a cool colour. But if you compare different reds (or yellows or blues), you'll see that there are warm and cool versions of each of these colours (relative to each other only). For example, cadmium red is definitely warmer than alizarin crimson (though alizarin crimson will always be warmer than, say, a blue).
Secondary colours are made by mixing two primary colours together: red and yellow to get orange, yellow and blue to get green, or red and blue to get purple. The secondary colour you get depends on the proportions in which you mix the two primaries. If you mix three primary colours together, you get a tertiary colour.
The proportions in which you mix the two primaries is important. If you add more of one than the other, the secondary colour will reflect this. For example, if you add more red than yellow, you end up with a strong, reddish orange; if you add more yellow than red, you produce a yellowish orange. Experiment with all the colours you have - and keep a record of what you've done.
But you'll find that there'll always be an instance when the colour you want simply doesn't come ready-made, such as a particular green in a landscape. Your knowledge of colour mixing will enable you to adapt a ready-made green to the shade you require.
The advantage of buying a premixed colour is that you are sure you are getting the same hue each time. And some single-pigment secondary colours, such as cadmium orange, have an intensity that's hard to match from mixed colours.
Browns and greys contain all three primary colours. They're created by mixing either all three primary colours or a primary and secondary colour (secondary colours of course being made from two primaries). By varying the proportions of the colours you're mixing, you create the different tertiary colours.
Mix a primary colour with its complementary colour. So add orange to blue, purple to yellow, or green to red. Each of these makes a different brown, so once again make up a colour chart to give you a quick reference to refer to.
Mix some orange (or yellow and red) with a blue then add some white. You'll always want more blue than orange, but experiment with the amount of white you use. You can also mix blue with an earth colour, such as raw umber or burnt sienna.
