I know a lot of people who like to paint their values first in black and white and then apply color to their values. I've never found an ideal way of doing this, but I've learned a couple tips that help it work out a little better.
First off, I don't let any of the values get too dark, except where I want things to drop completely into shadow (ie. black) In this case, I lightened the "grayscale" image before applying any color to it. You can see the original values I painted on my blog. I usually don't spend a ton of time working detail into the values because I often have to paint in all that detail again when I get to the colors.
The reason I don't often use it is the pain of dealing with the value shifts when colorizing. If I already spent a lot of time working out the values, I don't want to work all that out again. If the values are very detailed I sometimes feel like I'm painting the whole thing twice. However, this process suits some people's patience and way of thinking so hopefully it's worth sharing anyway.