If you cross the street with the light, check the sky before taking an umbrella, and add the right amount of cream to your coffee, you're making decisions based on information derived from your own personal color space.
Your eyes are delivering that information. They're impressive instruments, more sensitive to the infinite number of colors in the spectrum than photographic film is, but not as reliable. Film records those parts of the visible electromagnetic spectrum that are present, whereas the colors you see are influenced by cultural and individual expectations.
More
Computerworld
QuickStudies
Also, your personal color model has no objective standard. Try to describe a color to another person and you realize how difficult it is to translate from your color space to his.
This ambiguity led the Vienna-based Commission Internationale de l'Eclairage (CIE) in 1931 to develop a color space based on data from colorimeters, instruments that can accurately measure specific wavelengths of light. Based on an 1857 model by James Maxwell, the CIE's color space uses the primary colors of transmitted light: red, green and blue (RGB). This prevalent model is additive: Add red, green and blue light and you get white.
A second dominant color space model uses reflected light. This subtractive model attains white by subtracting pigments that reflect cyan, magenta and yellow (CMY) light. Printing processes, the main subtractive users, add black to create the CMYK color space.
The CIE's XYZ (representing red, green and blue) color space measures two components of color—hue (the dominant wavelength, such as red or green) and chroma (the color's purity). But the XYZ color space doesn't match perceived color differences, nor does it describe brightness, the perceived amount of white in a color.
In 1976, development of the CIELAB color space and a 3-D model allowed the inclusion of brightness measurements. The CIELAB color space remains the most accurate way to measure and reproduce color and is supported by scanners, computers and presses used in high-end print shops.
Targeted Functionality