Marbling fondant is a great way to add extra color to your cake. I would choose either two color from the same color palette for a subtle effect or two totally different colors for a bold effect. Rolls your two colors of  fondant separately into long ropes, than twist them together like a candy cane, from here roll your fondant into a smooth ball; now your fondant is ready to be rolled out to icing your cake.  If you want a more marbled look just repeat the steps again. I prefer just to use two colors of fondant, some people use three or more. The look will be different each and every time you marbled fondant.