I just happened to think how easy it would be to switch the position of the nozzles in the holders. Might not fool a diesel user, but quite likely a gas driver.
Ok, that was a mean thought. Don't try it
Any color under the sun is what I've seen, Green, Black, Red, Yellow. I read labels that's the only sure way.
BP uses green handle covers as their default for most pumps since green is one of their corporate colors you'll find that most BP stations will have all green handles on the gasoline pumps.