In an LED, a wafer of semi-conductor material is doped to create a PN junction and when current is passed through this junction, light is created by the annihilation of electron/hole pairs. The material used in the construction of an LED's PN junction determines the energy of the emitted photons and hence the LEDs colour. LEDs produce narrow range of wavelengths in the visible spectrum that we perceive as single pure colour.
White LED light is created in a similar way to fluorescent lamps, using phosphorescent and fluorescent materials to re-emit part of the light in a wider spectrum, creating white light. This light has a high colour temperature as a consequence of this method and is very suitable for vision applications.
The graph below shows the spectral output from a standard white LED, which is sometimes known as the 'dead penguin' graph!
More about LEDs: