Painting the glass can be very simple or overly complicated. I prefer very simple goto your local craft type store find some cheap water based acrylic craft paint generally runs 1$ or 2$ for a 4oz bottle. you'll need to thin the paint with a little water then either brush it on or use a foam roller. As stated before this is much easier if the tank is empty . These paints are non toxic and water based so no special masks are required I still wouldn't want to get any in the tank but it works well and if later on down the line you decide you don't like it it's easily removable with a razor blade it peels of in sheets.
Used this on my tank photos of the final result are in my Signature.