The natural color space used by display monitors matches the way the human eye perceives colors. Cones in the retina of the human eye contain three pigments, each sensitive to either red, green, or blue light. Each color, though, can have a range of appearances based on its richness (saturation). In the figure below, each box contains only one "color" (hue), but illustrates a range of saturation values:
In the RGB representation, adding white light to the image will change all three of the components. A representation based on hue, however, is independent of lighting conditions--a red object has a red hue whether it is dimly or brightly lit.