[logicrook]

No, I meant some pigments won't mix well because of other physical properties. The only example I can think of from the top of my head is in the Quiller, Color choices. It is mainly concerned with watercolors, and some colors mix uniformly (so blue+yellow -> green) and some look like an emulsion (so not green, but yellow with blue bubbles inside). Which can be interesting, because the mixing of colors is thus done by the eye instead of the paint, and is in fact one of the major painting effects used by impressionists.