How thick will your granite layer be? What are you going to use to hold it in place if it is on top of concrete? It sounds very strange, the gravel will constantly be kicked off the concrete as you walk, because it won't "bed" in any underlying layer of sand or dirt.

gravel on concrete is a natural skating rink...people will slip and fall. Try a layer of quikcrete on top tossing in all kinds of colored gravel. That would give you a lot of room for creativity.

Why not use acid stain to color the concrete? It can be quite attractive.

see here for examples

Here is a link that might be useful: acid stain examples