I would say, lets start at the beginning,..
1) Clean lines. By this, I mean, to make sure that your colors live only where they are supposed to on the model. This is an important skill, and you seem to have a good grasp of it so far. For me, nothing kills a model faster then seeing the flesh color from the arm bleeding over onto the models tunic, or the silver from a helmet also painted too far down onto the strands of the hair.
2) Try painting flat first. By this, I mean straight, flat, raw colors,.. straight from the pot. This is going to give you more of a "cartoonish" look to your work. But that's ok. This is just a starting point. Once you have mastered this (in conjunction with clean lines), then you can move to the next steps of mixing custom colors and/or shading and highlighting. But for now, lets just stick to basics,..
3) Black lining. By this I mean to paint or "draw" a thin black line wherever 2 colors would otherwise meet. If the blue sleeve would touch the silver gauntlet,.. put a small black line there to separate the colors from each other. If the green pants would touch the brown boots,.. put a black line there as well. This is going to add to your "cartoonish" look. And you may eventually want to remove it from your paint style. But for beginners, it's a good way to "train your eye" as to where the separation points between colors should be.
4) Don't forget the eyes!!. This one gets overlooked a lot too,.. but, in my opinion, it helps to bring a sense of completeness to the models. Steps are as follows,..
-1)Flesh color the whole face.
-2)Put white in the general eye area. (doesn't have to be perfect at this point, just in the overall area)
-3) Place a small black dot in the white of each side of the face. (placement is important so they aren't googly-eyed or what have you. Colors can be done, but at this scale black works just as good)
-4) Go back to you flesh color, and make a small rainbow like arc over each eye section, centering the arc on the black dot. And then the same thing underneath each eye. Basically, you're using the flesh paint to "trim the eye into existence". Or to put it another way,..
Don't try to "paint the eye into the flesh"
Instead,.. "paint the flesh ~around~ the eye"
This process will save you much headaches down the line, and keep you from wanting to pull your hair out.
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Once you are comfortable with the results you get from these first steps, then we can move you on to more "gritty/realistic" type paint jobs. There are, of course, other techniques to learn (many of which have been suggested). But I feel that this is a good place to start, and should get you the basic fundamentals anyways.
Just my two cents,.. since I'm not exactly a Rembrandt either *grin*