Fill an ice cube tray with water. Invite the children to place drops of yellow food coloring on one side of the tray and red food coloring on the other side. Freeze until solid. Serve the kids a clear drink like water or lemon-lime soda and place two cubes of yellow and two cubes of red inside the glass. Have the children observe what happens when the ice cubes melt and the colors mix. What secondary color appears?
Use play dough to mold a volcano shape inside a baking pan. Make sure to form a crater opening at the top that is a couple inches deep. Place a small amount of baking soda inside the crater. Pour white vinegar inside a small pitcher and invite the kids to add drops of red and yellow food coloring. To make the volcano erupt, slowly pour the colored vinegar inside the volcano crater. When the vinegar and baking soda combine, they cause a chemical reaction that forms a gas (carbon dioxide). This makes the volcano bubble and fizz down the side of the mound like lava.
Trim the ends off a white carnation flower and a stalk of celery. Place each into a glass of water. Put drops of yellow and red food coloring to each glass to make a brilliant orange. Over a period of time, both the flower and celery will absorb the colored water and turn a new color. This happens because as the stems absorb the water, the veins of the plant carry the color upward.
Place about an inch of milk into a shallow bowl. Add a drop or two of yellow and red food coloring in the center of the milk. Next, add a couple of drops of liquid dishwashing detergent to the glob of colors in the center. Watch the colors explode. This happens because milk is mostly water along with proteins and fat. The soap weakens the chemical bond that holds the proteins together and carries the color off to the sides so the molecules (ones that hate water) can reach air.