You will need:

Wax crayons in many colours, including black.
Heavy, smooth cardboard from a file folder etc.
A scratching tool (a comb, knife, crochet hook, or fork).

Now do this:

1.  Colour the cardboard with all colours of crayons except black. Press hard and make a thick layer of colour. Crayon in patches or stripes of colour until the whole cardboard is coloured.

2.  Now colour over the first layer with a thick layer of black crayon.

3.  Scratch a picture or design through the black so that the lower layer of colour shows through. It looks lovely.