Dianne, those deep, rich colors have fascinated me, also! Every artist has a different way of mixing colors and achieving certain effects. One very important point to remember is that dead color, i.e. straight black out of the tube, straight brown, etc., will not produce rich, shadowy color as well as warm color with some cool in it, or cool color with some warm in it.
To get an idea how I do it, here are two links to posts on greys and blacks:
http://forum.portraitartist.com/showthread.php?t=5572
http://forum.portraitartist.com/showthread.php?t=5156