Raindrops refract light in a similar way as a prism, thus the colors.
![Drawing showing light refraction through a prism and also through a raindrop.](https://d9-wret.s3.us-west-2.amazonaws.com/assets/palladium/production/s3fs-public/styles/full_width/public/thumbnails/image/wss-rainbow-raindrop-refraction.jpg?itok=80YTDWIJ)
Detailed Description
Just why do rainbows have colors? It is because of the the raindrops refract the light from the sun into a color spectrum. A prism does the same thing.
Sources/Usage
Public Domain.