So what is colour?
colour or ( US ) color
1. a. an attribute of things that results from the light they reflect, transmit, or emit in so far as this light causes a visual sensation that depends on its wavelengths
b. the aspect of visual perception by which an observer recognizes this attribute
c. the quality of the light producing this aspect of visual perception.
What thats really means
The eye sees the colour of an object as white light is shone upon the surface of said object, which then reflects particuler colours and absorbs others. It is the reflected light - or wavelength, that is picked up by the eye. Therefore a red ball
is reflecting red light and absorbing all the other colours.
Within the eye, the retina contains receptors called rods and cones. The rods react to brightness, whilst the cones react to colour wavelengths. There are three types of cone; red, green and blue. Each colour has a different wavelength; blues, greens and violets have shorter wavelengths and reds, oranges and yellows have longer wavelengths. When coloured wavelengths fall on the retina, the nervous system and the brain interpret the signals as colour.