I've been having trouble "On-screen" blending for quite some time now. For those who are confused with what I mean. On-screen blending is placing down a color and a following secondary color. Then, selecting one color and mixing it in with the opposite color (Usually the eyedropper tool). Usually when mixing, I set my Fill down to produce less pigment. The problem that always happens though is that my blending always comes out muddy and incorrect. Here is a Couple of images I have to prove this.
I'm not totally sure what you're talking about, but it sounds like you're using the wrong colors? Pick different ones. Try looking at pictures of people in different lighting and people in real life to get a better idea of how they look in shadow. Formulas usually don't work 100% of the time.
Muddy color is poorly chosen color. there really is no such thing as muddy color. It is operator inexperience. How much painting from life have you done? if the answer is none or very little you will always struggle with color and color mixing.