When I did my last overlays, I had a dark walnut on the bow/limbs and a pecan stain on the overlays. What I did was epoxy the overlay blocks on, sanded them to the shape desired and then used a wide brush to stain the bow all the way to the tips and then went in with a fine brush to hit around the overlays and the overlays themselves. But the brush on masking is a good idea also, that stuff is called liquid frisket (I do watercolor painting and use it often) and it's available at most craft/paint supply stores.