Ian, don't be afraid of the darks.
That just came up (really, I couldn't help it!) as I was about to say this: If you know the handprint site, Mr. MacEvoy has a page about how to ensure brilliant (or no mud) colors. Huge amounts of detail (as per usual
), but basically, you want to mix the color, then lay it, without 'fuzzing' the fibers of the paper more than absolutely necessary
So, either mix the dark and lay it with single strokes (very dark washes), or you can glaze, with single strokes on top of completely dry layers (as Sylvia explained). A wonderful-beautiful, user-friendly sable brush (I love my da Vinci flat #20 for washes!!! and my Escoda #20 for tighter corners!!!) makes it so much easier, but it can be done even without perfect tools.
If you get worried, simply tell yourself this one you're about to do is just the first of a series; you're experimenting. That's what I do, to take the pressure off.
When you do get there, the result is going to be great! And think how much you're going to learn along the way.