Generally, HID lights have an expected lifespan of around 3,000 hours or 90,000 miles (most people consider them a lifetime bulb)...
The color of the HID bulb is related to its Kelvin rating...for example, the purple HIDs are 14000K and the white bulbs are 4300K...I do not think that the Kelvin rating affects the longevity of the bulb, but I do know that the higher the Kelvin rating, the less bright the projection will be...For example, the 4300K bulbs are much brighter and give you better visibility, where as the 14000K bulbs are very blue/purple but they are not as bright as the 4300K
I personally love the higher Kelvin bulbs as far as looks are concerned...I have a silver Audi and and right now the stock HIDs are a medium blue color that really looks clean