(It turned out a little gritty. Not incredibly impressed with it, but it was fun mixing colors.)
Next up: Olive oil fireworks:
Just mix about a tablespoon of oil with a few drops of different colored liquid food coloring, stir well and pour into a clean jar of water.
Finally, baking soda and vinegar volcanos:
Spread baking soda in a pan, drop food coloring on, and then have a wee person use a dropper of vinegar to make miniature volcanoes.