I am assuming you don't have an airbrush so, get a good quality paintbrush and mix either powdered or liquid/gel food coloring with vodka or water (I like vodka). Then paint on the darker portion, feathering at the edges into the lighter area.
You will want to play with your coloring/vodka consistancy until you get the desired effect, practice on paper to see how it paints on before attacking the cake.
thank you...i forgot to mention it will be buttercreme not fondant....can i still paint the buttercreme?
or you can also buy those canned air brush things at your local walmart they dont spray as good as airbrush but it would work
thank you all!