RYB model

The RYB model stands for the triad red, yellow, and blue as primary colours in subtractive colour theory. It is a model still widely taught in art circles, even when the CMY model would be a more useful and satisfying choice.

(Image embedded from Wikipedia on 4 January 2011)

Depending on the choice of colours, a typical RYB triad works well on one side of the colour wheel (eg by mixing good greens), but is doomed to produce duller secondary colours on the other side (eg by mising dull oranges and violets). The bright secondary and tertiary colours you may see on a typical RYB colour wheel are not reproducible with just a single RYB set.

The split primary model offers a practical solution to address above problem, albeit it increases the number of primary colours to six (ie, two sets of primary RYB colours).

Want to know more?

Wikipedia - RYB color model
Wikipedia offers a bit more information on the RYB model in this page.