OR!! get yourself a cheap set of earth-tones pastel chalks at Micheal's or Hobby Lobby and some sand paper, and make your own pigments. Take your time and don't sand too hard or the powder won't be fine enough. And remember it doesn't take very much powder at all for the effects, so you can make a little and blend colors as needed.
I'm not certain of the number - anything fine, maybe a light medium. Sand paper is cheap. I'd recommend some trial and error. When I did it I just had all kinds of various grits around.
Oh, another tip. Get a small funnel and coil your sandpaper in side it (you may need to trim it) and then you can stick the end of the funnel into a container and just swirl the stick around the inside of the funnel. You'll need to tap out the powder that builds up after a while, but this method is a lot less messy.