I didn't paint the back on my most recent build. I used a 'frosted glass' vinyl on this one. The wall behind it is a light green, so the wall color is muted, but shows through. It's still dark enough that any cables or plumbing behind the tank aren't noticeable unless you're looking for them.
I went this route after having issues with paint on glass being scratched on a a previous build. The film seems to hold up better for me.
Amazon looks to have window film in other colors, but I'm not sure how dark they would be.
On the tank I did paint, I went with black. It really made the colors in the corals and fish 'pop'. I also painted the PVC black and the filter box I used for my beananimal drain was black acrylic. It made everything along the back wall of the tank disappear.