A thousand years ago, when Celestia banished Luna from Equestria and sent her to the moon, she was charged with three tasks. She originally was in charge of raising the sun, and showering the land with rainbows. But, with the moon being an additional task, she had to hand down the responsibility of rainbows. Celestia entrusted the Pegasi of Cloudsdale to make the rainbows for her from them on. For the first dozen years, we were given powerful unicorns to help create Spectra. Spectra is pure pigment, pure color. Everything is full of Spectra, but you can’t just harvest it. You can never separate color from an object. So it was made artificially with magic