When I first set the tank up it just had the white wall behind it, which looked okay but didn't hide the cables and got a bit boring.
Then I used a piece of brown cardboard for a while, which looked suprisingly good, and particularly suited the colour of the water when the driftwood was first leaking tannins.
Last weekend I added black card on top of the cardboard. It looks a lot better now, but it does show up the brown algae (hopefully the Otos will take care of that when they arrive) and any bits in the water a lot more.
I only background that I considered buying was a plain one with varying shades of dark blue.