You let the lighter base color dry completely before masking to spray the second color. Mask carefully, spray some clear over the masking edge so if there's any bleed it's clear. Then spray the second color, and remove the masking material before the color has dried. I usually only let the paint tack up briefly before carefully (because the paint is still soft) removing the masking. If you wait until dry the edge may be difficult and the tape may have done some etching into the base color.