that's pretty much what I do. I do all the gray tons under the color and that does alot of thew blending tricks for me. I was alway taught to do it that way because my teacher told me 'if they abuse the hell out of the tattoo and all the color fades on the 30 years down the road, atleast you still gave them a nice blk and gry tattoo.'
I do for the color to blend them together. I'll whip out the red for instance if i'm moving on to orange because if I pack solid orange over the whipped out red it fades smoother from one to the other