This unit investigates the origin of colors in materials, including gasses, liquids, and solids. "Color in gasses" includes experiments that explore the color of fireworks, the origin of smog, the cause of rainbows, and why the sky is blue. The "Color in liquids" section includes investigations into why plants are green, the color change of acid/base indicators, why objects change color when they rust, and how glowsticks and fireflies produce colored light. The section dealing with "Color in solids" examines how colors are produced by the interference of light using an audio CD, how UV light can be detected using fluorescence, and how light emitting diodes work. Investigations are also performed with materials that change color in response to temperature (thermochromics), which are often used as temperature indicators. The unit concludes with an investigation into materials which change color in response to the application of an electric current (electrochromics).