That's some good info, thanks guys. It's weird. I would've never thought that putting different colors on different layers would've blended with each other correctly. I just figured the top layers would obscure the lower layers, but it sounds like with the blending modes, it might work. I'll give that a try.
When I do greyscale first, I'll use a few different colour layers: - one with the blending mode set to "colour" for the base colours - one on top with the blending mode set to 'normal' for blending and more colour variations - one more on top for final touches