The rainbow is a refraction of the light of the sun using the water vapor as a prism.
The light being sent from the sun is mirror-flipped when it goes through the water vapor. The rainbow, then, doesn't have enough distance for the light to settle out such that red is on the bottom.
Light doesn't have mass, at least, not on its own. Light's mass only manifests when it interacts with something. It's effective mass is relative only to the energy it carries.
One other point.
Consider this:
The rainbow is a refraction of the light of the sun using the water vapor as a prism.
The light being sent from the sun is mirror-flipped when it goes through the water vapor. The rainbow, then, doesn't have enough distance for the light to settle out such that red is on the bottom.
Light doesn't have mass, at least, not on its own. Light's mass only manifests when it interacts with something. It's effective mass is relative only to the energy it carries.