Standard white light bulbs often have a label on their packaging telling you what colour they are. Using labels gives you an instant idea of what colour they are but it is not standardised and the descriptions vary from manufacturer to manufacturer. To ensure consistency there is an international standard for measuring the 'whiteness' of the light; this is the colour temperature.
Colour temperature is measured in Kelvin (K) after William Thompson who was the first baron of Kelvin. It has a typical range of 1000K to 7000K with the lower the number the warmer or more orange the light and the higher the number the cooler or more blue the light. This is most easily demonstrated on a sliding scale:
Typical colour temperatures:
|6000||Daylight||Daylight white fluorescent and compact fluorescent bulbs|
|4000||Cool White||Typical metal halide HID lamps|
|3500||White||Standard fluorescent and compact fluorescent lamps|
|3000||Warm white||Halogen bulbs are usually slightly cooler than incandescent bulbs|
|2500||Warm white||Traditional incandescent bulbs are typically around this value|
|2000||Flame light||A match or candle will give out a very warm or orange light|
The introduction above just concentrates on the relevance of colour temperature to artificial lighting but it is a very wide ranging topic covering many areas. There is a very good article here that explains it in great detail if you wish to know more.