I'm with AndreZA, darken your colors. (Don't just spray black on top.)
I was always told: There is no black in the natural world, just darker values of colors. Highlights are the same way, unless it's wet or has a hard shine...it's never white. I have broken those rules, sometimes I get lucky...sometimes I pay for it,

.(In all honesty, colors are still a big pain in the neck for me)
I do my shading with a darker version of my base color. In this case I darkened most of this with transparent red mixed with my base color, I added just a little blue to my shadow color for the darkest areas.
View attachment 1244
Watch Airbrush Tutors video on flesh tones, he talks about making different values of the same color.