As long as it's dry by the time you fill up your aquarium - then anything but oil based paint and mold and mildew resistant paint should do.
I personally use flat white spray paint for the inside of my canopy's and three to four coats of flat black spray paint in the back of the aquarium. Of course, take precautions when using spray paint.
One thing to never use in a canopy is a mirror. Instead of reflecting light, it absorbs it.