The only background paper I have used on marine tanks was a paper graduated from light blue at the top to dark navy at the bottom. Those seascape backgrounds always seem too busy to me, and ultimately detract from the real centerpiece of the tank - the organisms. The back of my current 220 gallon tank is painted black and is fine. One reason I went away from the backing paper is that inevitably salt creeps between the backing and the rear of the tank, which ruins the look.