You got it in your first sentence. Experimentation is a great part of this software and without it you'll never be able to understand what is going on and what can be done (which is virtually endless). So I'd say; just try it with some gray scale file, and a colored file blended by the gray scale file. If you can't get it to work, post a file or screenshot or image, and help will be available.
I may add that the whites in the gray scale map will be 100% of the colored file, black will be none, and all tones in between a semi-colored terrain.