Some colors go well with one another, and are often found in nature together. Some people like a uniform look, some don't. Some people try to recreate a natural look, while others decorate to their own taste. I would google/yahoo/what ever search engine pics of cichlid tanks (seeing they are mainly rock decor), or any fish tanks, and see what you think. Or you could just put the rocks in your aquarium and see if you like it. Every aquarium I have owned, I have redone countless times til I got what I liked.
Use whatever you want. It's your tank, YOU have to like it.