The earth's atmosphere acts as a prism which bends the light into all of its rainbow colors. The one that bends the most, the top-most color of the rainbow is red, and this is the color that predominantly lands on the moon and comes back again as the regolith reflects that red rainbow color the Earth's atmosphere produced.
https://www.youtube.com/watch?v=uzorkQxyc-g&t=82

Basically you're seeing the Earth's rainbow on the moon.