How easy are hair chalks to use on very short hair? Are they ok for already coloured hair?

I'd never even heard of hair chalks but a quick google and I found this...

http://www.thebodyshop.co.uk/content/hairchalks.aspx

It seems very easy to use and washes out too.

I'm quite dark-haired and quite fancy the blue!

I can answer the first question.
My hair is short, white/grey. I used purple & turquoise (purchased from hairchalk.co.uk.
Very easy to apply and I did 2 shampoos & 1 condition then applied my Pro Naturals argan oil before drying and styling.

You can ask hairchalk via their linked email whether it is OK on already coloured hair.