LEDs generate monochromatic light. The color of this light depends on the wavelength of the light, which is defined as the distance between successive points of equal amplitude and phase on a wave.
LEDs create a narrowband stream close to the ultraviolet, the visible and the infrared light frequencies. The coloring of this light is defined as the dominant wavelength. Through the addition of phosphors on the LED chip, other colors can be produced, e.g. white or pastel colored LEDs. LEDs are monochromatic (single color) devices. The color is determined by the band gap of the semiconductor used to make them.
Red, green, yellow and blue LEDs are fairly common. White light contains all colors and cannot be directly created by a single LED. It is a Gallium Nitride blue LED coated with a phosphor that, when excited by the blue LED light, emits a broad range spectrum that in addition to the blue emission, makes a fairly white light. White LEDs enclose all spectral colors of visible light. It is possible by using a group of three LEDs (Red, Green and Blue) and controlling the current to each LED to yield an overall white light.
Another possibility to create white LEDs is to put several LED chips of different color into a common LED housing and mix them so that the resultant color is similar to that of a white light output.

Examples:

Semiconducting Material	Color
Aluminum Gallium Arsenide - AlGaAs	Red
Aluminum Indium Gallium Phosphide - AllnGaP	Red, Orange, Yellow
Gallum Arsenide Phosphide - GaAsP	Red, Orange, Yellow
Indium Gallium Nitride - InGaN	Green, Blue