No longer an issue, but I'm still kind of curious.
The answer to your question was that a glass overflow was going to be siliconed to the back of the tank. You can't paint the back of the tank first, because silicone won't bond to paint. This would make the silicone visible through the glass, which is ugly IMO. The only options are to paint the inside or to place a piece of acrylic on the back inside the tank. I went with option #2.
Take a look at the pic and you'll see what I mean...how would you make the background blue on this tank?