In the past, I have used tipp ex to cover the light stain left by blu tac, although the walls have to be white, obviously. You could always go to b+q and spend a quid on a tester pot to paint over any marks when you leave. Or youcould try the trick that a previous occupant of a friends room at uni had done. At some point they had made a hole in their wall, I would expect by accident. So they had written a lovely note saying what a nice time they had had in the room, and hoped the next occupant had such a good time, and stuck it over the hole. My mate left it there with a small comment confiming such a good time was had for the next occupant. As far as I know, the cleaners never touched it and the hole is still there.....