When you see a rainbow, you are seeing light reflected off drops of water similar to when sunlight bounces off raindrops. The colors are a result of refraction, or the breaking up of white light into its colorful parts. 

Rainbows are actually circles of colors, in a spectrum that many people remember best as ROY-G-BIV – red, orange, yellow, green, blue, indigo and violet. The ROY-G-BIV spectrum was described by Sir Isaac Newton, but there are many more colors within a rainbow, and each person sees the colors of a rainbow differently. 

For more about rainbows, check out this Discover Magazine article.