the color layer simply allows you to lay down a quick color pallete that is tied specifically to values beneath it, but in my opinion its poor acting as sole means color over a greyscale painting. Alot of artist use a combination of layers to get the job done. Overlay layer seems to be the meat and potatoes, but the key is applying the color that has the same value as whats in the underpainting (this is more true with a color layer, as overlay can affect the value as well)...like using a brown or purple for your dark greys, for example. Painter makes this job hella easier because you simply rotate the color wheel to choose the color without changing its value.
In addition to overlay, there is screen, which will allow you to apply color while also lightening as well. This is a great witha soft brush to add that glow affect. Then there is multiply which is the opposite - adding color while darkening.
Most everyone seems to always have a top normal layer for some hard changes and corrections.
going about this whole process has been a real hit or miss for me, so normally when I render in greyscale its simply for the sake of getting the composition just how I want it and then simply paint over with color on a normal layer. This too also seems to be a common approach it seems
"I was born; six gun in my hand; behind the gun; I make my final stand"~Bad Company