MH bulbs

I currently have a 265 gallon planted tank with three (3) 175 watt 5500k MH lights. I purchased the lighting system (hanging horizontal pendant lighting) last spring (May/June) and have replaced all the bulbs within the last two months. My local fish store had a Hamilton bulb (175 watt 5500k) in stock and when I plugged it in I noticed a difference in the color of light, I originally thought it was because my other two bulbs had not been replaced at that time and they were faded somewhat. However last weekend I replaced the other two bulbs (not Hamilton) and noticed the color is indeed different. They are also 175 watt at 5500k. I do not know who the manufactor is, but shouldn't they be the same color of lighting since all three are 175 watt 5500k? Is there a difference in the brand of bulbs, and also what is the usual life span of these type of bulbs? Any thoughts???