The black walls are either black styrofoam that they stick inside the tank or black paper/sticker that they stick outside the back of the tank.
I think blue might be a nicer color for your consideration. More punchy color contrast against anything reddish.
By the way, water parameter and diet is what makes the color of your shrimp attractive. The background can only enhance a little. That's all.
Cheers,
Bookmarks