For planted tanks i've seen a few colors being used...
I've personally done black, white and 3D backgrounds on my planted tanks all with good results...
Then for the more adventurous ones out there, i've even seen people make a light box behind their tank, and add a blue light shining through it, which adds some serious depth when looking at the tank, while giving it a very natural sky looking effect...
There has also been one time i've seen it done with a more orange bulb, which gave the tank a slightly sunset look to it...