With my recent obsession for brightly coloured hair it was just about time I discovered hair chalking. All you'll need is artists pastel chalk, a bit of liquid styling product (I used leave-in conditioner) and hairspray. Spray the product on the part of hair you want the chalk to stick, and paint the hair with the chalk. Set with hairspray. It will come all off with the next shower, and as you see it worked quite well on my dark hair.
(And, to be quite honest, a lot easier than that overpriced colour bug I bought)