Paint is only available after upgrading Nook's store to Nookington's. When you buy paint, your roof color will be changed to the color of the paint you purchased. Paint is not a furniture item, as when you buy it Nook uses it to redecorate your roof. However, through hacking, it is possible to set up a paint can in your house, which is simply the paint can seen in the store. It takes up 1 space, and you can walk over it, like shovels and other tools.