One way to practice is to use watercolor markers (Crayola brand work well) on a RC proof print. You use the marker as a color source and simply rub a wet brush against it to get the tone, which you then apply to the paper. It helped me work through the degree to which I wanted the final print colored as well as the process. I found that sublety was elusive at first but after seeing the image with color on it, I could go ahead with the final print with more confidence.