My favourite starting point for colour mixing is the colour triangle rather than the more familiar colour circle. I think it’s easier to understand and makes remembering complementary colours simple.
(If you don’t see the video above, click here to see it on my Vimeo channel.)
The fundamental rule of colour theory for painting is that there are three primary colours: red, blue and yellow. The second rule of colour theory is that mixing two primary colours togethe creates secondary colours, that is purples, oranges, and greens. On a colour triangle, the three primary colours are at the points, and the three secondary are on the “flat bits”. All you need remember initially are the three primaries, because you can always mix two to remind yourself what they create.
The other reason I like the colour triangle so much as it makes it easy to remember complementary colours. These are colours that make one another look brighter, and also desaturate each other (make them less intense in colour). On the colour triangle, complementaries are the colour opposite, so Blue + Orange, Red + Green, Yellow + Purple..
The first color triangle is attributed to the 19th century French painter Delacroix. A notebook of his dating from around 1834 has drawing of a triangle with the three primaries written in as rouge (red) at the top, jaune (yellow) on the left, and bleu (blue) on the right, plus added the three secondaries as orange, violet, and vert (green). Delacroix adapted the triangle from a color wheel in an oil painting handbook by J.F.L. Mérimée, a painter he knew.
(Source: “Colour and Culture” by John Gage. Thames and Hudson, London, 1993. Page 173.)