i wish i had a sample of my work to post, as it would really help describing this problem, but...here goes.
when i'm painting in oil, i don't have a problem creating smooth transitions between tones when i'm using monochrome (lately i've been using prussian blue mixed with varying degrees of white) but i seem to have problems creating the same smoothness with different colors, i.e. a recent experiment was prussian blue for the dark tone, yellow ochre for the middle and white for the highlights. i'm just not getting the same smooth transition between colors when i'm blending. this sucks because i am trying to use actual colors. this is sort of difficult to describe, so i hope someone can shed some light!