It may also be the voltage regulator; HID regulators have two phases of operation, initially there is a fire up high voltage phase and then there is a maintenance/operating phase that keeps the bulb running. On my TL my right side would fire up fine but after a few minutes it would turn bright pink and sometimes flicker... the bulb itself was ok, but the regulator was bad.
Hopefully it's just your bulb but you may want to get it checked so you don't waste money on new bulbs that may get taken out by a faulty regulator/controller