When I created my first cover, it was a very busy design. But I was proud of it, and the concept I had that related the front and back covers. Then I read a blog about what makes a cover good. Mine broke all the rules. Crushed, I decided to ignore the blog advice.
But it just kept nagging at me. Challenging me. Don't you hate it when that happens? So I decided to try to make a cover that fit the rules. And I liked it better than the original!!
One of the rules, the one I remember best, is the rule of three. Use only three colors, or color families, in the cover. So red, blue and yellow, for for example, can be used, in varying shades. But then there's no purple, green, brown, black, white, teal or orange. Just pick three colors. You can use lighter or darker shades of those three, but no others. It makes an appealing design.
But don't take my word for it. Try it!