Color

Aliases used by humans for specific frequencies of electromagnetic radiation around 700 to 400 nanometers. (light)

If a certain object does NOT reflect a certain frequency it is being applied the according "color" adjective.

For example:

660 nanometers is refered to as red. If an apple reflects all frequencies BUT this, then people say the apple 'is red'. Violet light is electromagnetic radiation with wavelengths of 410 nanometers and red light has a wavelength of 680 nanometers.


 * http://acept.la.asu.edu/PiN/rdg/color/color.shtml