I'm pushing it with the B for Bow in rainBow, but here we go!
It's National Find a Rainbow day.
Who's that guy with the mnemonic name? ROY G. BIV! He'll tell you the of colors in a rainbow are red, orange, yellow, green, blue, indigo, and violet.
Rainbows often show up after the rain. The arc shape looks like a bow (the kind used to shoot an arrow). This is where the name comes from, as far as the Internet knows.
What's your favorite song mentioning a rainbow?