Colorants, in the form of polymer additive pigments (inorganic and organic) and dyes, change the look of a given plastic to the human eye. Aided by light the colored plastic now appears green, blue, red, yellow, black, white, or any one of a 100,000 standard hues in industrial use today. It is stimulated by light received by the observer's eyes. Light is electromagnetic waves, and the human eye sees light between 400 to 700nm (nanometers) in the visible wavelength spectrum. A plastics part color stimulus is one part illumination and one part observation.Observation keys to the influence of the plastics part on the fraction of light it reflects across the wavelengths from the light source to the eye of the observer. Thus color is in the eye of the beholder.
The CIE 1931 Standard Colorimetric System in use today was established at the 1931 Commission Internationale de L'Eclairage (CIE, International Commission on Illumination, www.cie.co.at; Vienna, Austria) conference as a means for precisely defining the color of light.