I've just made a great discovery. It's the L'oreal Double extender mascara thing, with white on one end and whatever other colour you want on the other.
Use the end with the white on it in an upward motion to stiffen your "peach fuzz" up, and then wait a few seconds for it to dry, and then apply the darker stuff. It works great for a 5- o'clock shadow look and really looks authentic if you do it right (make sure you don't smudge it or actually get any on your face). If you mess up, I find it useful to have a Q-Tip handy, moisten an end, and use that to clean up your mistakes, so you don't have to start all over.
I'm not sure if it comes in a waterproof variety, but that would probably be better, cos people sweat, it rains, etc.
Do try it out...
I'll put a picture up when I do it again, so you can see the effect's realism.
Anybody else have fun, cheap ideas for facial hair that have worked in your experience?