I've put one on my smallest tank, It was to stop the sun from hitting the tank (algae control)
But I think it did work to brighten up the tank too.
Black on the outside, blue facing into the tank, but only on the end that faces to window.
Edit: The tank's positioned so i can't see through the side anyway, so i don't lose anything by covering it.
Sometimes I like to look through the sides of my tank so I can see right at the back and just to have a different feel of the tank so I wouldn't do it.