I like what you have done with your tank, but if you are going to paint the back the recommended paint to use is krylon fusion paint. As your tank is already full you would find it a lot easier to get a plain black background from a lfs and stick on, unless you have easy access to the back of your tank to paint it.
u have a few threads so im gonna put it here. these are pictures of my tanks a 40g, 20g long planted, and 10 temp gallon growout. all painted black back and 2 with brown and 1 with black gravel.