Yellow's 90G Upgrade

[yellowworld]

This is my problem. I wanted to paint the back of my tank from the beginning but it somehow slipped to the wayside, now i'm kinda stuck for what I can do.
1. Painting the back is a possibility still but my only concern is that you'll still be able to see into it.
2. I thought about a piece of acrylic in the overflow but I don't think the colours will match the paint even if perfectly matched beforehand (see picture - the darker is on the outside the other on the side and the last inside the overflow)
3. I am now thinking that I can get a full sheet of thin acrylic to place inside the tank the full length of the wall. BUT there are two issues with that - how to get it to stay flush with the back wall and there are a few points of my rock work that is against the wall ( I cant move it very easily)

Here is the colour problem (they are all the same - left to right, inside overflow, outside tank (darker), inside tank (same as overflow)



Anyone got any thoughts or other suggestions?