Changing the cover color

How can I change the color displayed for the cover of a Basic Bloom Book or standardize the cover color in a Collection? Currently, the Collection chooses a random color out of four available and I’d like to have each of my Collections be a different color. This will not be for print purposes, but will be used for a cellphone app. Thank you.

If you will be using Reading App Builder to make an app, the cover for the books will be given via an image you set under the RAB “Cover” tab. I can’t get RAB to work for me just now, so I don’t know what happens if you don’t provide a cover.