Use the color picker to get the color of the wall, set the color transparency to about 190, then make a new layer use the paint bucket on the new layer, then cut the parts you don't want colored.
quick version:
I'm fairly new to Paint.net but I thought I'd post what I've don so far:
6/6/08
I don't know what I'm going to do with this :? , it's really more of a animation feat then graphic.
6/3/08
I made this to use as a banner for a site I'm making. I use a variation for my signature (not included in this post).
6/2/08
I made this image for a banner contest, I'm working on a animated version.
more images coming soon...