||Whatever the venue, whether it's mail order pieces, catalogues, or book covers, it's been proven thousands of times over and over, across the board, that color sells far better that plain black on white. Color covers are simply more attractive that black and white covers, and make a book more weighty and professional.
In most cases, your book will not even be taken seriously unless it has a clor cover. you hand a buyer a book with a black and white cover, that buyer is going to think your book is small-looking, that you're small press, some mom amd pop operation making homemade books. You want a bigger look, a cover that says professional, one appeals and attracts, and that means a color cover.
There's a trade-off in doing color covers as opposed to black and white, and that is that color covers cost more produce. But it is a trade-off that should not only be made, it is one that should be considered automatic. A given. Forget the savings a black and white cover can give. Whatever the savo\ings might be, it's not worth it.
Color cover are the only way to go.