Bulbs are marked with a number followed by "K", indicating the color temperature.
K stands for Kelvin, an absolute unit of temperature in the international system. Please note that since this value is absolute, we note 273K and not 273°K.
Any heated body emits electromagnetic radiation (light), even if it's not necessarily visible to the human eye.
Kelvin works like Celcius: the hotter it is, the higher the value.
However, on light bulbs, the opposite is true: a bulb at 1800K (~1527°C) is said to be "warm", one at 5000K (~4727°C) is said to be "cold". Which just goes to show that we shouldn't leave serious matters to the marketing people.
As temperature increases, so does radiation energy, according to Boltzman's law of 1879.
Low energies are infrared and red colors, and high energies are blue colors and UVs.
