I made a post a while ago with information on creating an endless set of randomized color schemes that can then be used in cover generation. Basically, I found
a website that allows one to generate text from a regex template, and created a template whose result could be pasted into the file where calibre saves color schemes.
You can ask the website to generate as many iterations from the template as you want. I think I have a few thousand themes saved in calibre, which I refresh every few months or so. I've attached a screenshot of my themes list just so you see the end result.
I'm also attaching a screenshot of some custom covers I made for journal articles, shrinking the size to display as many as possible. Looks like I got 230 in there, and no two use the exact same colors. Of course, I use some typesetting hacks and I'm running from source with tweaked copies of the default styles, which helps with cover diversity too. But the colors are all generated from regex.