That has to do with the colour mixing method. ArtRage uses an RGB colour mixing method. We opted to go for this mixing method for familiarity for users of digital art programs.
There are pros and cons to this. If you're used to real world paint mixing it tends to work in a different way to most digital painting software. Artistic colour mixing is something we've had suggested however, and we do pay attention to suggestions by users when considering future upgrades.
Hope that helps clarify things!