The Kelvin (K) is the unit of color temperature. 4100K is thebrightest, most natural white light similar to that of daytimesunlight,and therefore the HID color temperature used most in the OEM automotiveindustry. When you look at the light coming directly out of anHIDheadlight, it possesses a bit of a blue or purple hue which most peoplerecognize as the expensive, elegant look characteristic of HID's. For this reason, some HID kit manufacturers produce 5400K, 6500K,8000K,etc. bulbs that give an even more distinct blue hue but at the expenseof overall light output. For comparison, most halogen headlightshave a color temperature around 3200K which gives them a "dingy"yellowish appearance compared to HID's.
A: That's up to you. Our most popular kits are the 5000k and 6000k, but we've also had some match their headlight color with their car color. Take at the chart below and see what suits you best.