As for which COLOR background... depends on what fishes you will be keeping? Like most of the replys posted, I prefer black as my main background, but I've also noticed the colors on some of my fish, look 'different?' or stand out more in my other tank that has a blue background?!!
For example, i've noticed some datnoids (not all!) will display a more vibrant golden/yellow color in a tank w/ Blue background. Throw that same dat in a tank w/ Blk. background, the datnoid darkens up, and looks all blotchy with patches??... I've noticed w/ Peacock bass too, they show more vibrants colors in blue, than black backgrounds. Alot depends on fish's true colors, body patterns, etc. Some background colors will bring them out, and other color backgrounds will make them fade off, trying to blend into the tank environment. You see where I'm getting at...

Try experimenting first before making a background color permanent.
Something I used to do is take actual photgraphs of my tanks, everytime i did a 'change up' in the setup... (change of gravel, substate color, background color, change/adding of fish, adding/removing of decor., etc.) Then put those pictures in one of those photo sheet protector things w/ the sleeve. And use it to visually compare the differences & impact the 'change' you made on the tank. Basically like a "Before & After" shot of the tank. You'd be surprised to see how much it would make or help to actually see a visual.
Anyways, good luck and happy keeping!!
