This is great if you are painting seawater....
You will need:
1/4 mug warm water
6 teaspoons salt
3-4 drops food colouring - blue or green or a mixture of both
Mix all 3 ingredients together well. Paint with the mixture. Once dry the water evaporates leaving the colourful salt! You could also try sprinkling a few extra salt grains on the 'paint' while it's still wet for 'bubbles'.
