If you have access to an air brush or some type of sprayer, I have something you can try. I've seen it done but never tried it myself. You can cut out a piece of cardboard in whatever shape you want the transition to be (eg. a "V" shaped transition on a bow limb), and suspend it about an inch or so above the limb (sitting on stacks of pennies or something. Then when you spay it, the transition will fade into the other color, rather than have a distinct border.
There are probably easier ways, this is just a painting technique I've seen that I thought might be helpful.