There are many sources for natural pigments, some of which work well as food colorings and some of which do not. Challenge your class to bring in food items with vivid colors, so you can evaluate which ones work best as food colorings. Chop or mash the different foods and simmer them in an equal quantity of water for 15 to 20 minutes. Strain the solids from the resulting dyes and pour them into transparent containers. Have your class identify which foods made the best colors. Compare them using strips of white paper, or pieces of bread or eggshell.
Find a reference work such as Harold McGee's On Food and Cooking at your library and identify which pigments are in the foods you've used to make the colorings. Ask your kids to group their sample dyes accordingly, comparing chlorophyll green to yellow and orange from carotenoids, blue and purple from anthocyanins and the vivid reds and yellows of betains. Ask them to compare the colored test strips made from each pigment, and draw conclusions about the nature of the dyes. For example, would carotenoids from carrots make a better yellow than betain from golden beets?
Cookbooks often include the instruction to add a large amount of salt when boiling vegetables to help preserve their color. Another common tip is to add a pinch of baking soda to the water, or in some cases vinegar. Have your class add small amounts of vinegar or baking soda to several of the dyes and record the changes, if any. Chlorophyll, for example, is very sensitive to acids, while anthocyanins will alter color in the presence of a base, such as soda. Ask your class to speculate what effect this might have on cooking the vegetables.
Easter eggs are a common way to use homemade food colorings and they can also be a kid-friendly science experiment. Choose several of your best colors and divide them equally between beakers. Have your class add 1/2 tablespoon of vinegar each to one set of beakers and leave the rest as they are. Have your kids decorate the eggs by soaking them in heated dye and letting it cool. Ask them which set of dyes made the best colored eggs and have them speculate why. Explain that eggshells are made of calcium and the acidic vinegar helps the dye adhere to the shell.