I'll copy a nice explanation from Skidmark to explain it.
To achieve a marble look, you will need a few things:
Plastic bag from a grocery store
A sheet of paper
Marbleizing usually works best with pearlized paints, and close to the color you are going to use for your background. You want the effect noticeable, but not overwhelming. For instance, pearlized blue for the marble, backed with ultramarine blue is very striking! Now, you can do other colors for a freaky effect, like red pearl, on a black background. It's easy to get creative, but very easy to get ugly. Be sure to test out your color combo's before doing the real thing!
Take the paint you are going to use for the marble effect, and pour out about a teaspoon on a sheet of thick paper. Now take the grocery bag and wad it up into your hand. Simply dab the wadded bag and dip it into the paint. Take a few dabs on the paper to get rid of the excess paint, then simply dab the paint in various places on the body. All that is left now is to fill in your background. Simple!
base colors I used in my body are:
Faskolor faspearlred
Faskolor faspearl black
faskolor faspearl black
To make the red and gold marble I used the same paints mixed with a little black to get a darker tone, the faspearl black is backed directly with regular black.
this is a outside pic, that shows better the real colors.
by the way, you will find many nice painting tips in Skidmark's site.
http://www.cptnet.com/ken/faq.html