You might consider rattle-cans of spray paint as an alternative.
Otherwise about your only alternatives are to 1) Either mask the areas or 2) Brush them freehand. The quality of the edges if you do it freehand will, of course, depend on you. I can't paint with a brush and would not try it myself, however your abilities may be much better than mine.
Either way, if possible start with the lightest color and work toward the darkest color.