Lamps marked BU are base up, HOR are horizontal, and lamps marked U are universal. MH lamps are rated for 10,000 to 20,000 hrs lamps life. In the aquarium trade they are discarded after 4,000. So if you hang your lamp in the wrong position it would cut the lamp life short but your discarding it before the 10,000 to 20,000 hrs it is expected to burn anyway. As far as color in the comercial side when a lamp company sets up to run 41 k lamps, the first lamps will have a blue tint or bright white and last of the run before they retool will be more yellow. Next time your in a gym look up and you will see what I am talking about. I wish the industry would do a better job of keeping lamps as they are made in the same order.