Summarize the following:
Pick a few non-permanent markers that look like a good bruise color. Dark reds, blues and purples are all good choices. Use a brownish yellow (or yellow and brown) marker for the outside edges, as well.  Make sure to not use metallic or glitter markers, which will look obviously fake when you get them onto your skin. Highlighters give a great yellow-bruise look as well. Use a highlighter instead of a yellow marker, if you've got a highlighter on hand.
Use basic colored markers.