I tried to view this file after downloading it, but unfortunately couldn't get the video to show. Is it perhaps a divx video or another codec? I can certainly install that codec if that's the case.
Otherwise, if you'd like to send me a painting file with a block of each colour so I can see the original colours, an area of the colours mixed together and information on what tool you used to mix the colours, I can look at it that way and let you know if what you are seeing is standard behavior in ArtRage or not. Hope that's of some help!