i have plants on my backgrounds. you can buy some which are doublesided, so you can change the background to suit your mood!! i think the plants backgrounds add an illusion of depth to a tank. let us know what you decide! they are very cheap and can get them on ebay new and cut to size in various heights.
I like blue with normal lighting as it adds a little somthing to the tank I think. I don't like the backgrounds with stuff on it, is distracting and doesn't make your tank itself stand out, black or blue makes your tank/hardwork stand out more.
blue background in saltwater and freshwater tank. The Freshwater tank was taken when I was redoing the tank and it doesn't look like that now, looks much better just haven't taken an updated photo.
I have a plain black backdrop on two tanks, and like the way plain black brings out the color of the fish. One of my tanks has the back painted black and it looks way better than those plastic/paper background.
On my small community tank I did a little experimenting with my airbrush on a piece of matboard for the backdrop.. blue on top... fades to green.. then fades to black towards the bottom. Looks OK, but I'll probably change it soon. With the lights I am using in the tank, you can barely even see the green.. the whole thing just looks kinda black most of the time. Certain times of the day when the light is right, you can see the green. The only reason you see green in this pic, is because of the flash of the camera...
I used mostly green because my LFS has a discus tank set up with a dark green backdrop and it looks awesome.