Methylene blue should stain it nicely (and your shirt, hands, face, and wherever else it gets). I'd check on the internet from a dye or chemical company and see if you can get it in bulk. Dilute it with water (stain darker than you want, it will bleed, but I can't say how much), wear 600 pairs of rubber gloves, and apply it with an old rag. Or, if you can pirate an air gun or buy a cheap air-driven paintgun and air compressor, spray it on the cement for a very, very efficient and even look. Let it dry for a couple days, then soak in several water changes.