Mixing Your Own Colors

Colour theory can seem complex and daunting, but once the basics are understood, it begins to make sense. Four main terms are used to categorize colour: temperature, intensity, tone and hue. Temperature describes whether the color is warm or cool, with the reds, oranges and yellows being warm, and the blues, greens and purples being cold. Intensity indicates the brilliance or purity of the colour. Tone assesses the relative lightness or darkness of a colour. Hue is the general name given to a colour such as red or blue.
The primary colours - red, yellow and blue - cannot be mixed from other colours. When mixed together, they produce a range of secondary colours - orange, green and purple. Tertiary colours have elements of all three primaries and tend to echo those colours found in nature. Neutral colours are mixed using equal amounts of the three primaries. By varying the proportions, a range of browns and greys are produced. One colour can be changed significantly by adding white (or water) to lighten it producing tints or by adding black producing shades both ends of a tonal range.
Colours which fall opposite one another on a colour wheel are know as "complementary" (red and green) and have a unique relationship. Mixed together, they tend to neutralize one another but placed next to each other, they appear to increase in intensity.
The best way to learn about colour theory is to experiment with a limited palette and range of combinations.

...from "The Instant Artist"; Ian Sidaway, Collins & Brown, Ltd., 2001