A base coat of violet is first. Try not to make the bruise a
perfect circle. It needs to look irregular to look realistic.
Soften the color as you go out so it doesn't have any hard edges.
To make the bruise look really deep, add a little dark blue or
even some black.
Adding red will lighten the bruise.
To make a bruise look old (healing) add yellow to the surrounding
bruise. Blend well.
That's it! Very simple and cool. I would have to say that the
older bruises looked the most realistic because even after the
initial shock of seeing it, it still looked very real. Why not
give this simple technique a try.