The Colour Wheel
A good place to start a discussion on colour is the basic colour
wheel.  You can buy one in an art store, but it’s better to
paint your own, using your own paints. Start with the basic
primary colours (say, Cadmium Red, Ultramarine, and Cadmium
Yellow).  
Gradually blend tiny portions of blue to the yellow, red to
the blue, etc to get a gradual transition into Oranges,
Greens and Purples, the secondary colours.
Then, moving in towards the centre of the circle, gradually
add some of the colour diagonally opposite each of the
colours. Colours diagonally opposite on the colour wheel are
called “complementary” colours. When mixed, they have
the effect of “neutralizing” or “greying out” each other.
Now you are looking at a range of “Tertiary” colours. As you
get into the centre of the wheel, you tend towards black.
In theory, you should be able to get every conceivable colour from the three primary
colours. However, there is no such thing as a perfect red, or the perfect yellow or
blue. Instead, colours are made from pigments, metals and minerals, each of which
have their unique characteristics. Cadmium Red for instance, is about as true a red as
you can get. But it is on the “orange” side of the spectrum. Similarly, Ultramarine is
everyone’s basic blue, but it has a purple tinge. As a result of these imperfections,
you will find that it is not possible to get a completely comprehensive range of colours
from three tubes of paint. However, you will be surprised at just how close you can
get.

Colour Temperature
By convention, we generally describe colours in the Red/Yellow/Orange sector of the
colour wheel as being warm, and those in the Blue/Green sector as cool. “Warm” and
“cool” are relative terms however, we can speak of a “cool yellow” (like Lemon
Yellow) or “warm green” (like an Olive or Sap Green). Usually, warm colours will tend
to “come forward” and cool colours will tend to recede into the background. So, in a
painting, if you want to make something recede into the distance, one way to do
this is to use cooler mixtures as you go further into the background. Conversely,
adding reds for instance will appear to pull the foreground closer to the viewer.