Definitely put a background, it will hide any wires or hoses behind tank. As far as color it is your choice. I had black before and now I have blue, I prefer blue myself. I used blue vinyl that I got from eBay, it us applied just like window tint.
I say go black. That is what always looks best IMO. I also use window tint from the hardwear stores. Its cheep and you will not get salt creep or water behind it..
Either color you pick...clean the tank really well and a few coats of Krylon Fusion will give you a good background...I did blue on my tank but black would be a more defined edge of the tank.
I used to have a black background but it really limits another potential "view". I love to peek around behind my tank and see the corals and critters from another view!
I like it black because it makes the colors of the fish and coral pop. Sort of like using black sand does to corals. But congrats!! Choose what is good for you.