From what I have been told, spray paint is not a good thing for a tank because of the additional chemicals that are put in the tank to make it come out of the can. While I know it isn't the CFC's like was the case in the past, I cannot remember what exactly it was that is added to the paint in order to make it thin enough to come out of the small nozzle.
Thinking logically, which I try to do, I am sure that there are certain paints that you can use, probrably with a brush, that you can paint decor with and be safe for water. Of course, the questions would be what paint will not dissolve in water and which ones will not leech chemicals into the water. Maybe even looking at a store that sells aquarium decor would point out what type of paint is used and then you can pick some up at a store and do it yourself.
By the way, welcome to the forum! Feel free to ask anything you are wondering since I can guarentee you that someone on this forum will either know the answer or be able to point you in the right direction!