You can paint the outside of the sump with almost anything. It's not going to be in contact with the water and should be fine. I painted the back of my display tank with a few coats of plain latex wall paint, using a foam roller. I bought one of those "color sample" cans. I did 3 coats with very light sanding between coats to make sure it stuck. When the final coat dried, I covered it with clear film to protect against chips and wrapped the film about an inch around the sides. It's been running for a few years with no problems.
The other thing you might consider is taping sheets of dark craft board over the outside of the glass. I've used this for years to block out light when raising fry. Works very well, but the board will get wet and warp so it will need to be replaced every so often. I never used silicone to "glue" it on because as the fry get older I need to pull the board away to give them a little more light.
Both are cheap and effective solutions and neither needs to be submerged.
