Best color boils down to individual taste. That's a 17 gallon tank right? 14" high? I wouldn't paint anything on the back but make different color backgrounds that you could tape on the back. Is the wall behind the tank light-colored? You might try no background at all. Or use a mirror behind it to give it a sense of added depth. It also depends on your 'scape: if the back is going to be covered in plants, a background becomes a non-issue. If you have enough space behind
the tank you can experiment with different contoured shapes that look like
dark rock, roots, etc, and use a light source to illuminate it.
Painting a background on the back glass is a one-way street, unless you want
to tear the tank down to change it, which is more trouble than it's worth