I prefer the black or natural sand that looks dark with specks of light but mainly darker (what option is best can also depend on which fish you are keeping).
For the background I prefer the graduated blue color vinyl, but the darker blue if you were to paint it because Black sand and black background is sooo dark.
Black Eco Complete isn't totally black, here you can see it in some detail. FTS is a stock black back, it comes that way from the factory no choice.
Here is the graduated blue back ground with Garnet sand. Not a good pic but you get the idea of the background.
here you can see the sand better, most accurate color is in the center between the fish.