Color temperature describes the hue of a light source. It's based on experiments conducted by physicist William Kelvin. He heated carbon which gave off different colored light as it grew hotter.
HID (High Intensity Discharge) lights are high-powered headlamps with warm and cool colors. If HID bulbs are placed in the wrong headlamps there will be glare in some parts of the beam pattern and insufficient light in others, making them dangerous and often illegal.
Warm colors are colors in the red to white range, including yellow and orange hues. The warmth of a hue varies in inverse proportion to its Kelvin rating --- warm colors have a lower Kelvin rating than cool colors. This does not mean that HID lamps with warm colors are less bright than colors with a higher Kelvin rating.
Cool colors include bluish whites, blues and indigo shades. They have higher Kelvin ratings than warm colors. HID lamps with cool colors may produce less usable light than bulbs with warmer colors.
A color temperature of 3,000 Kelvin indicates a yellow light. This rating is sometimes used instead of halogen fog lights.
4,300 Kelvin indicates a white light with a yellow tint. It is fairly close in color to sunlight. In HID lamps, this color gives the most visible light. As it has he best performance, it is generally chosen by car manufacturers because of its high performance.
6,000 Kelvin is almost pure white, but has a very faint tint of blue. HID lamps with this rating have a very high performance, although they don't give off quite so much visible light as 4,300 Kelvin lamps. This color is very popular as an aftermarket option.
HID lamps with an 8,000 Kelvin rating give off a pale blue light. They offer less visual light than warmer colors.
HID bulbs with a 10,000 Kelvin rating give off a deep blue light. The brightness, and the performance of the lamp as a light source is significantly lower than a 4,300 Kelvin lamp.
At 12,000 Kelvin, an HID lamp gives out a violet light. Visibility is significantly poorer than with other colors. Such lamps must never be used for night-time driving.
