There are many ways of getting marbled effects in rubber stamping projects. One of my favorites is to stamp rainbow or multi-colored stamps pads onto a piece of kitchen foil and to use this to ink the stamps. There are an almost endless number of ways that this can be varied. I use this technique to squeeze the last few inkings from an old pad, as it doesn't matter if the ink isn't even on the foil and any gaps can be filled in with other colored inks. A light misting of water encourages the ink to bleed and blend which gives an interesting effect.
This is a great technique for creating stunning effects with the minimum of effort and really shows off a line stamp to its best advantage. My tip is to stamp up a number of images to keep for last minute stamping emergencies!