Whats the difference between white, black and silver backed MR16 light bulbs?

These guys deserve a review just because of how expertly packaged they sent my fragile and delicate T5 tubes. I will admit not having high hopes of a courier being able to deliver these in one piece but they were snugly bubble wrapped in a very sturdy cardboard tube. Fantastic fast delivery.

BLT suggested an alternative light bulb for one that had been discontinued which meant I didn't have to replace the fitting, very helpful, thanks