Robert is correct - it probably relates to the lighting we apply to the canvas.
Press <F5> to turn off the rendered lights.
In the real world there's no such thing as a white piece of paper. Even if the paper was a perfect diffuse reflector with no flaws, the colour is still determined by the lighting. You wont get a perfect light source that will give you a perfect distribution of chromatically perfect light across your perfect diffuse piece of paper.
So in ArtRage we add some chaos to the creative process with a rendered light source across imperfect paper.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"