Colour theory
A set of primary colours is a small, arbitrary set of pigmented physical media (ink, oil, crayons, etc.) or lights. Usually, the set is of three different colours.
The French School of Painting in the 18th century created the RYB Model for subtractive mixing. The clues to determine more accurately a group closer to the ideal of primary colours were found during the 19th and 20th centuries. Blue was replaced by Cyan and Red by Magenta. The CMYK model was created. This model contains four colors instead of three. This is because, although the mixture of cyan, magenta and yellow must produce black, the result is not satisfactory and supposes an unnecessary expense of pigments. It is cheaper and you get a sharper black using pigments of that colour.
Find more information about the old RYB model following this link:
There are also some videos on the Internet to help you remember primary and secondary colours. Even thou it seems childish, this sample video will help you with colour mixing.
Finally, two colour wheels. They solve the same problem, but they do it on two very interesting ways.
