Briarjumper, practice, lots of practice
I use oil or alcohol based aniline dyes, lay the dark down where I want it with a small patch of cotton cloth, then use the lighter color on the balance of the bow(clean cloth of course), get a good solid dye there too, then gradually come out of the light up into/invading the dark, dragging it into the lighter color a little at a time, blending/fading as I go... switching cotton dye rags with clean, used single color, or used blended colored-cloth, etc. as needed, along the way. Whatever it takes to get the effect I want. Sometimes some of the best finessing is done as the rags are beginning to dry out.
0000 steel wool can be used afterwards to help ease the fade of different colors into each other, to put 'highlights' along the edges, to remove color from the summer wood while leaving it in the porous spring wood and grain, or to fade the lighter dye into the natural color of the osage and bamboo. Some of these techniques are used on single color dye jobs, as well as dye jobs of 3 or more colors.
It's actually not that hard to do, and only takes a bow or two to get the hang of it. You can use test pieces too of course.