Since the widespread adoption of HID systems you will often see bulb color specifications with 3000K, 6000K, all the way through 12000K sometimes.
Kelvin, for those who aren’t big on Physics and thermodynamics, is a unit of temperature measurement.
It’s usually used for really high temperatures, for example things like the sun. The sun is actually about 6000K. We use kelvin units to measure the color of light. Different colors are created at different temperatures.
The lower the kelvin, the more yellow the color, the higher the kelvin, the bluer the color. 3000K is a very deep yellow, 5000K is white, 6000K on things get bluer and bluer.
Note that higher kelvin does not mean higher output. In actuality the higher the kelvin, the lower the Lumen output. When you get up into the 8000k on range you start getting about the same or less output as stock halogen bulbs.
If you want the most output, you want to look in the 4500K-5500K range. This is why all the HID kits I sell are 3000K-6500K and either OEM or Morimoto. I am not in it to sell gimmicky parts that don’t perform; I’m here to get the most performance out of headlight systems as possible.