High-Intensity-Discharge Lights (HID's) AKA Xenon Bulbs Or Lights - Which One is Right For You?

In the quest of customizing our rides, as well as getting better night driving visibility (and being noticed by onlookers), the most common aftermarket upgrade that auto enthusiasts do to their rides is replacing the halogen bulbs of the headlights for xenon bulbs, or to be more precise, High Intensity Discharge Lights (HID's).
The term "Xenon Headlight" is actually is a misnomer. There are several types of HID lamps, and those without xenon follow this sequence when turned on: A sudden bright flash of light, followed by the lamp turning dim, then slowly becomes bright as it reaches its normal operating condition. The xenon gas is only used to speed up this process and provide instant light upon ignition of the lamp.
Upon purchasing HID's, there are a multitude of Kelvin temperatures to choose from, usually starting from 3000K, all the way up to 12000K or even higher. Be wary of this as it is a common misconception that the higher the Kelvin temperature, the brighter the light. This is NOT TRUE.
The Kelvin temperature is also known as color temperature. Simply put, the color temperature, measured in degrees Kelvin, determines the color of the light output, not the brightness. The lowest color temperatures used in HID lights usually have yellowish hues, and as you go up the color temperature chart, it goes from white, to blue, to purple. The stock halogen bulbs are around the 3200K range.
So if you are to choose which one is right for you, well you first have to determine your application. Are you installing this for show, or are you looking to get the HID lights which can provide the most useable light output? If you are using this for show, then go ahead and purchase any color temperature you'd like to get to produce the desired look. But if you're looking for brightness and usable light output, then you're limited to a small range.
To give you an idea, manufacturers such as BMW and Mercedes Benz use 4300K to 5000K HID lights on the models they release which have HID's as stock options. These are the models which usually come with the "Bi-Xenon Headlamps" or "Xenon Adaptive Headlights" tags. Why do they stick with this range? Because it is this these color temperatures which produce the greatest useable light output.
If you'd still want to mix "show" with "function", for safety reasons, HID's with color temperatures above 8000K should not be used as the main headlights but instead, as an accent to be used as foglights. Higher color temperatures tend to reflect off fog and rain, and therefore do a bad job in illuminating the road in poor conditions.
And going back to the example of Bimmers and Mercs, the reason why they come with halogen auxiliary fog or driving lights is because lower color temperatures are known to be better at penetrating water molecules. These compliment the HID's in poor road conditions. Low color temperature HIDs (yellow hue) should be able to do work the same way if used as fog lights.
4300K HID's have about 3100 lumens of output, which is more than 3x the light output of halogen bulbs. This is the color temperature which give the most output. The light is fairly white, with a slight hint of yellow when reflected off the road surface. OEM HID's come in color temperatures from 4300K to 5000K.
6000K HID's have about 2900 lumens of output, which is about 3x the light output of halogen bubs. The light output is white, with a slight hint of blue.
8000K HID's have about 2500 lumens of output, about 3x the light output of halogens. The light output is blueish white. The most popular aftermarket HID conversions are from 6000K to 8000K color temperatures.
10000K HID's have about 2300 lumens of output, which is more than 2x the light output of halogens. The light output is blue and approaching violet.
12000K HID's have about 2000 lumens of output, which is more than 2x the light output of halogens. The light output is blueish purple.
Though you should be warned about retrofitting HID's into headlamps meant for Halogen bulbs. These are not without fallbacks due to the scientific differences between HIDs and Halogens. But there are many different HID headlamp units available for various vehicle makes and models, making this upgrade very doable for almost any of your rides.