Well, this might be a nice present for x-mas to everybody who wants to be a streetartist, but is afraid of the consequences that his illegal deeds might have.
Rather than leaving your mark on a wall, leave it on a long exposure. The Light Paint Can is a super-bright UV light housed in a very convincing spray paint can.

The Light Paint Can comes with letter stencils and a reusable UV-sensitive backdrop so that your light paintings will last longer for film or video.  Get it here