Have you ever heard of a "marble" finish for concrete?
Yes, although the technique is really only suitable for use over small areas. A marbled effect can be produced by painting grain patterns on a glass or smooth plastic sheet with a mortar of colored cement. Any desired mineral-pigment color can be used. The mortar mix must have the consistency of heavy cream. As the colored mortar becomes tacky it must be backed with a thin layer of ordinary portland cement mortar, again of a rich and heavy consistency. As this new layer becomes tacky it should be carefully transferred to the concrete surface for which the finish is desired. The base concrete should either be freshly stripped from a form, or otherwise mechanically or chemically pretreated to ensure bondability.