Some interesting points I found while Googling this.
—The sun will always be at your back when you look at a rainbow
—The shadow of your head will be at the center of the circle drawn by the rainbow. For illustration, see the page linked by erichw1504. In the first photo you can see the photographer’s shadow.
—The colors from top to bottom will be red, orange, yellow, green, blue, indigo, violet (you may remember ‘ROY G BIV’ from high school physics)
—The violet edge is most intense, it gets a bit fainter as you move up to red
—A double rainbow has a second larger and dimmer bow surrounding the first. The second bow’s colors are reversed, from the ground up they are VIBGYOR.