I've almost completely eliminated bleeding by pushing my royal icing to the thickest possible consistency for the task at hand (the thicker the icing, the faster it dries) and allowing lots of dry time (if the technique allows) when laying light and dark icing next to each other. NOTE: Even if you've let a dark outline dry overnight, it will still bleed into a light color placed next to it if that light icing is very loose and takes a long time to dry.
So, when doing wet-on-wet or marbling (aka pull-through) techniques where colors go down wet next to each other, I use the thickest possible consistency that will still allow a smooth marble without leaving any tracks. Immediate dehydrating of the cookie can also help to set the colors faster.