DIY Glue and Food Coloring Suncatcher Tutorial from Babble Dabble do. This is a 5 minute craft (except for drying time) made from recycled plastic lids.
DIY 2 Ingredient Homemade Watercolor Recipe from Growing a Jeweled Rose. Really cheap and easy watercolors with ingredients you already have. Also, as mentioned in the post, you could use vineagar instead of water for added fizz.
DIY Painted Salt Sculpture Tutorial from Fun at Home with Kids. All you need is table salt and water. You then paint the dried salt sculptures with watercolors or food coloring. Display the sculptures or have your kids break smash them up.
DIY Tie Dye Tissue Paper Using Food Coloring or Watercolors. Wonderful way to introduce kids to tie dye without using real dye. Tutorial from Land of Nod here.