This form of art has been known for quite a while, I remember doing it as a kid and just the other day my mother was talking about how much she enjoyed making scratch art as a kid too (mind you it wasn't colorful :).
Making the colored background
- Crayons obviously (worked naturally)
For other "techniques" you'll have to rub the paper with a regular candle once you or your kids are done coloring
- Acrylic colors (worked like a charm)
- Colored pencils (worked like a charm)
- Water colors (failed)
Different age groups like to use different techniques (older kids prefer colored pens and acrylics while younger ones can go crazy with crayons) .
Painting with acrylics and waxing the paper with a candle is super quick so this technique is perfect for teachers making sheets for their students!
Making the scratch-able black overlay
Black (or any other color) tempera paint with just a drop of dish detergent (or soap) mixed in. This way you'll be able to apply the color evenly on the waxed sheets.
If your kids are sensitive or you don't want to use detergent you can mix tempera paint with egg yolk - it works out pretty much the same.