Here is a different approach to coloring your value studies that creates way more richness than just slapping on a color layer. I like to take a reference photo, stretch it across the canvas, and actually use it as an underpainting beneath my line work. It looks chaotic at first, but it forces "happy accidents" and gives you a complex color palette right from the start.

I will show you how to set up the layers, use the Glowing Edges filter to extract your lines, and paint into selections to turn that messy base into a polished piece. It takes a little longer than other methods, but the color depth you get is worth the extra effort.