With acrylics, it's all about layering: applying progressively darker (or lighter), very washed out shades of your original base color (i.e. red, red with a touch of brown, red mixed with brown evenly, brown-red, brown, dark brown, black-brown, etc). That's how you can blend contrasts, create highlights, accentuate shades, etc. The trick is to err on the side of caution and start with what looks like tinted water - just enough color to leave a subtle result, and then reapplying the shade, mixing with the next gradation of shading, and so on. You have to experiment with what can be a lot of fun.
Some resources that might help, and which have helped me:
http://www.amazon.com/gp/product/849652717...d=397PML4TNJ79N
http://www.amazon.com/gp/product/190257962...d=397PML4TNJ79N
Also, there are lots of articles within pF from very capable painters, such as these links:
On painting and shading reds:
http://www.planetfigure.com/forums/index.p...topic=10055&hl=
On painting faces:
http://www.planetfigure.com/forums/index.php?showtopic=1208
I am only a year into painting, and there are many more qualified painters on pF to help you, but perhaps this will give you a start.
Cheers,
Kyle