Marbling is a distortion technique in which colors are dragged across one another, mixing into each other. Like the effect you get when you drop a little bit of cream into a bowl of soup just before serving it and then flick a fork through it to create a decorative pattern.
There are two way of creating a marbling effect using Corel Painter 9. One is very simple and fast, using the Marbling Rake from the Distortion palette. The other, which gives you more options and far greater effects, is the Marbling Effects tool accessed through the Effects menu.