Colour temperature has been described most simply as a method of describing colour characteristics of light, usually either a warm (yellowish) or cool (bluish), and measuring it in degrees of Kelvin (K).

The “colour temperature” of a white light is a simple method of describing the colour characteristics of the white light. Most white light is not pure white, it has usually got a bit of a tint, usually either warm (yellowish) or cool (bluish).
When a shop asks you, “what colour temperature are you looking for?” often it can get a little confusing as we start asking ourselves, “Does light have a colour temperature?, why do we use temperature to describe the colour of light?” It is a term which can be used in the aquarium industry to compare the colours of different sources of light.
With this said, you can not compare a 10000k fluorescent light with a 10000k halide light as the output from the halide will be far more than that of the florescent.
It is all about thermal radiation. When a black object such as a piece of iron is heated up it changes colour. This colour depends on what temperature the object is heated too. If you look at diagram 1 you will see that the more blue in a spectrum the more energy in the wavelengths and in turn the higher the Kelvin.

When the object is at 0 degrees celcius it still gives out some radiation (kinetic energy). At -273.15°C there is no radiation. This point is referred to as 0°K (absolute zero).
When a black object such as a piece of iron is heated up, it changes colour depending on the temperature it is heated to. This range of colours is used as a reference for determining the colour temperature of a light (white) source.
Typical examples of Kelvin used within the aquarium industry:
6500k – Tropical – Warm light typically used for tropical setups. A kelvin rating of about 6500 is the best kelvin temperature for freshwater plants. They are often also used in shallow reefs for symbiotic zooxanthellae in corals. Anything deeper than 12 inches and the red part of the spectrum hasn’t enough energy to penetrate the increased density of salt water, often blue tubes will be used to supplement this lighting.
10000k – Reef White – The 10,000 kelvin lamps will achieve good growth rates but not quite as good as the 6500k at the lower depths. The increased colour temperature (more blue) means there is more penetration therefore ideal to be used in a marine tank between 12 and 20 inches due to the increased PAR at greater depths.
14000k – Marine White – A very popular option within the reef keeping community due to the increased blue in the spectrum (hence the higher kelvin rating). The wavelengths have more energy therefore have the ability to penetrate the denser saltwater providing corals at greater depths with the PAR required (depending on species). The growth under these tubes at shallower depths wont be as good as the lower Kelvin bulbs but there is less need (but still used) for supplementary blue lighting.
20000k+ – Blue – With the increased blue in the spectrum the 20000k or higher kelvin rating tubes contain the first spike in the PAR spectrum so it is ideal to use this type of lighting for stoney corals, clams etc and as a supplement for the lower Kelvin 6500, 10000 and 14000 Kelvin tubes. They help to provide more penetration, coloration and growth in those deeper tanks.
For more information on PAR please see our article- What is PAR
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