The question most people want to know is why there are different colour backs to the foils. The answer has to do with the finished piece, and only matters if you are using transparent glass. If you are using opaque glass that you cannot see through it doesn’t matter which type you use.
The backing should be of the same colour as finish to the solder lines. Since you can see through transparent glass, you will be able to see bits of the back of the copper foil, especially at certain angles. It would look peculiar to have a piece with a beautiful black patina, only to catch glimpses of copper when you are looking at it.
Therefore, if you are planning to patina your piece black, use black-backed copper foil. If you are planning on leaving it silver, use silver-backed copper foil. If you are planning to patina it copper, use copper backed copper foil.