it is after rain. The sun is always behind you and the rain in front of you when a rainbow appears, so the center of the rainbow's arc is directly opposite the sun.
Most people think...
the only colors of a rainbow are red, orange, yellow, green, blue, indigo, and violet, but a rainbow is actually made up of an entire continuum of colors—even colors the eye can't see!
We are able to see the colors of a rainbow because...
light of different colors is refracted when it travels from one medium, such as air, and into another- -in this case, the water of the raindrops. When all the colors that make up sunlight are combined, they look white, but once they are refracted, the colors break up into the ones we see in a rainbow.
Every person...
sees their own "personal" rainbow. When you look at one, you are seeing the light bounced off of certain raindrops, but when the person standing next to you looks at the same rainbow, they may see the light reflecting off other raindrops from a completely different angle. In addition, everyone sees colors differently according to light and how their eyes interpret it.
You can never...
actually reach the end of a rainbow, where a pot of gold supposedly awaits. As you move, the rainbow that your eyes see moves as well, because the raindrops are at different spots in the atmosphere. The rainbow, then, will always "move away" at the same rate that you are moving.