Xenon HID bulbs have become more and more popular with car manufacturers due to their increased visibility and aesthetic appeal. Rather than the boring yellow halogen look, a Xenon HID bulb will give off a pure crisp white light which mimics natural daylight at a 4,300°K temperature.
With time, a Xenon HID bulb goes through something called color shifting which causes the output to become more white and slightly higher on the kelvin scale by changing the complexity of the electrodes. If you have a bulb that that has gone out and you need to replace it we highly recommend changing in pairs so that the output it similar.
If you are in a pinch and do not want to replace both bulbs at once, you should go with a 5,000°K bulb. The color color temperature of a 5,000°K bulb matches closely to that of a one year old 4,300°K bulb. This will give both headlights a similar color without breaking the bank although we always recommend replacing in pairs since bulbs usually fail within a short time from one another.
Please note that the bulb’s color temperature keeps changing during its entire lifetime. But the changes that occur after the bulb’s first year has passed are not noticeable anymore, so even an older 4,300°K Xenon bulb can easily be replaced by a 5,000°K one.