Having studied radiometry, colorimetry, and the psychophysics of our visual perception, we now have the appropriate background to study the subject of color order systems. This is a subject that is often discussed on an intuitive level, but the concepts and the logic of color order systems can be much better appreciated if we have a proper knowledge of the physics of color and the psychophysics of human color perception. Therefore, we have delayed discussion of this subject until now. Color order systems are important in applications because they provide some practical solutions for many color problems in our daily life, such as how to specify the paint color we want and how to coordinate the colors of furniture. Color order systems are also quite important for the explicit expression of our theoretical thinking and understanding of how we perceive colors, such as the opponent-color processes.

Introduction

How many colors can we distinguish? The number is estimated to be more than one million [713]. How do we accurately communicate with each other about a particular color without actually showing a real sample? Obviously our vocabulary of color names is too limited for this purpose. A system is needed to order all possible colors according to certain chosen attributes in a well-defined manner so that any color can be specified by its attributes in the system. In principle, a color order system can be designed purely on a conceptual level [519]. However, for the convenience of practical use, most color systems are implemented as collections of physical color samples. This makes them easy to understand and easy to use, and means it is easy to make approximate interpolation between colors.