Many people have heard of the story of Noah's Ark. The inhabitants of the world are all wicked except for Noah, so G-d sends a flood to drown everyone else. It rains for 40 days and 40 nights, the water rising so high it covers the mountains. The only ones saved are Noah, his wife and immediate family, and the animals they took with them onto the ark. 
When the flood waters subside, Noah sends out a dove which brings back an olive branch. Now that they know the waters have receded and plants are starting to grow again, Noah and his family come out of the ark. G-d promises never again to bring another flood, and offers the rainbow as a symbol and a reminder of that promise.
In order to look deeper, let's switch gears. The human eye has two different overlapping vision systems: one that sees in color and another that sees in black, white, and gray. In daylight we see in full color, and under very low light we can only distinguish between light and dark colors.