Usually by adding white and brown (Burnt Umber normally). But you can use anything you like really, depends on the colour you want it to be:
white + blue, red, orange, violet or green;
fleshtone;
grey;
sand.
einions right, mix the black with a light tone of a colour that goes well with the rest of the mini. keep it subtle though, because if you have too many highlights, it\'ll stop it looking black
I tend to use black/blue and white mixed. I will go very sharp in contrast if I want the cloak clothing etc to look like new leater otherwise not so sharp.