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!
This might be because yellow ochre and prussian blue are near compliments: blue/ orange, and tend to create a neutral color that might not be blending well with the two you used to create it. I've never blended those two colors together but I can only advise to create "steps" of mixtures instead of just blending them on the canvas like I assume you are doing. For example a 25/ 75 mix of yellow ochre and prussian, then 50/50, then a 75/25, which would be the brightest before yellow ochre and use those to aid the blending process.
"A drawing is not necessarily academic because it is thorough, but only because it is dead. Neither is a drawing necessarily academic because it is done in what is called a conventional style, any more than it is good because it is done in an unconventional style. The test is whether it has life and conveys genuine feeling."- Harold Speed [[Sketchbook]]