I usually use a color layer and a mask to alter those types of things. Trying to fiddle the actual colors on the layer seems to always get tedious.

It helps if you get into the habit of putting your colored areas on different layers as well. Then you can colorize and adjust that layer only to get what you want. In other words, have your black and white drawing and then have the coloration of land, sea, whatever all on separate layers. Use masks to have the colors only effect the areas you want.

Hopefully, that makes sense. If not, I can put together a better explanation. It's pretty much what Jturner is saying I think except adding a mask into the equation. If you don't do masks there is a great tutorial on them here.