Whenever I've had HID bulbs that cut out it's because the ballast is bad. In fact I just replaced one of my ballasts about 3 weeks ago because the bulb would sometimes flicker on/off or just not even turn on.

The bulb being white doesn't necessarily make it bad, it might just be a different color temperature than it should be. But when you try swapping or replacing that bulb you'll know for sure.

 

The shift in color temp is achieved by putting different concentrations of sodium in the bulb. As the bulb's life progresses, some of the sodium is consumed and the bulb color will change.