Everyone loves colors—the colors of spring and summer, the colors of butterfly wings and rainbows, the colors of soap bubbles, and the colors from a CD. How are these the same? How are they different? Should we look to the same cause for what appears to be the same effect? The rainbow is arguably Nature’s most beautiful optical display. After a rainfall, the bow of colors can extend from horizon to horizon. The creation of the rainbow involves the physics of refraction and reflection and a geometry first explained by Descartes. This chapter features the study of colors by Isaac Newton and others who devised some wonderful investigations.