ArtRage currently doesn't use a real-world correct colour blending system. It uses a sort-of additive gamma-corrected digital system in RGB space.
In RGB space: Red + Blue = Magenta.
Red + Yellow = Red + (Red + Green) = orange.
Blue + Yellow = Blue + (Red + Green) = White (or grey)
With real-world oil paints, most of the above combinations produce slight variations of 'mud'
With subtractive inks in CMYK space, the results are different again.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"