I like the different colour titles and banners, you should keep it like that. Or at least if you make it all one colour, keep these as an available option for those that want it. What are you doing about the text/content description on the back of the cover? If you could just have it as basic as possible so it can be used for any set, that would be great.
But yes, keep the multi-colour sets, it makes everything stand out that lil' bit more.