This was recommended and sounds great, but chalk dust is very bothersome to
some students and teachers, so I'd go with the food coloring or liquid
watercolors. or colored inks.
Leslie in Framingham
use salt and rub with chalk to get the colors we want. You put the salt on
a paper plate and then rub 1 color to mix it to the right intensity. It's not
as real but it works.Place sand in a pie tin (or other flat
container) and rub colored chalk over it (on the side of the
chalk). As the chalk becomes dust, it mixes with the sand and
causes it to become colored.