Our future graffiti artist

You might have to click on the picture to appreciate this masterpiece that greeted me when I woke up yesterday morning, but I was happy the medium of art he chose to create this beautiful picture was in pencil. Contrary to my belief, pencil was not much easier to remove then, say, pen, especially on the slick paint. Crayons would have been an even better medium for him to use when graffiti-ing my door. After much scrubbing with a washcloth, so as not to remove the paint Jameson (and I) got it pretty well clean. Too bad the cleaning part was a lot of fun to him. Hopefully we won't have a repeat of this in the near future as he knows he is ONLY supposed to color on paper. However, the temptation is great, as it is to all children, for him to see how it looks on walls, tables, doors, you name it. How can I manage to foster his creative streak without allowing him to color all over my house?