On occasion I've used (probably gonna get reamed for it) Special Effects hairdye, since I have it around the house by the gallon anyhow. It's a veggie based dye, nontoxic, and I just put a blot of it on the top fur, not touching the skin at all. I only leave it one for maybe 3 minutes, then rinse rinse rinse. Depending on how groom-addicted your rat is, it can last a while (only lasted about 3 weeks on Spork....lasted about four MONTHS on Zero...)