I think spray paint would work, but as others have said that its propulsions might jet you away, however, I think the main issue is light, and how in space, everything is basically an absence of light. So you could get the paint on the asteroid but since there is no shining light on that specific asteroid, then no one could actually observe it.
What you need is a predictable rock occurrence, like Haley's Comet that is sure to follow a pattern and then have it be in an age that it can be observable by a timely occurrence of a star shining upon it...