"Rainbow of color" or "Rainbow of colors."

Although the rainbow above is a spectrum of colors, the word rainbow here is a metaphor for an abundance of good things such as the beauty of color and the delight of joy. The festival is filled with color and joy.

If it were an actual rainbow, we would not need to hear there were colors, as we would know that.

American Heritage (5th Edition) definition:

A diverse assortment or collection.

Compare the phrasing from the Shaker hymn Simple Gifts:

In the valley of love and delight

Why would we want to be in a valley with only one delight instead of a cornucopia of them? We would not, but the delight is the valley's specialty, and it has its generous share.