Test which color lasts the longest. Pour equal quantities of milk into four same-sized clear containers. Add the same number of drops of food coloring to each container of milk. Stir until the dye has completely dissolved. Take a photograph of them. Cover each container with a lid or piece of plastic wrap and refrigerate. Leave them undisturbed and check them in 24 hours. Take another photograph of them to see if any of them have faded or changed. Repeat this process every day for five days. Note any observed changes.
A variation of this experiment is to vary the kind of milk you use instead of varying the color. Use whole milk, 2 percent, skim, soy milk, got milk, almond milk and rice milk.
Put five drops of blue food coloring into one-half cup of milk. Repeat with red food coloring in a separate cup of milk. Cut the ends of two celery stalks or the ends of two white carnations and place each one in the cups. Set them aside. In 24 hours, look at the two cups and observe what happened to the stems and stalks. Note if one color traveled up the stem or stalk farther than the other.
Pour room temperature whole milk into a shallow bowl or pie pan so that it is 1/2-inch deep. Place one drop of four different colors in different areas of the milk. The drops should look like four points of a square. Gently drop one drop of dish soap in the center of the four dots. Do not put the dish soap on top of any of the food coloring droplets. Watch as the color moves and the milk is transformed.
Make colorful, malleable plastic with milk, vinegar and a few drops of food coloring. Pour a pint of milk and a few drops of food coloring into a saucepan on low to medium heat. When it begins to simmer, stir in 2 tbsps. of vinegar and stir the mixture. It will curdle and form lumps. When the mixture has completely divided into solids and liquid, remove it from the heat and allow it to cool. Strain the solids from the liquid and knead them into shapes or press them into a mold. Leave it to dry for a day or two and observe as it becomes hard like plastic. Experiment with different colors.