Like a sound, smell, touch, etc., color is a characteristic of sense, which we can acquire by seeing with our eyes. The sense is acquired by the fact that light comes into our eyes. In other words, the cause of seeing a color is light. Therefore, to express the sense of color more strictly and quantatively, it is necessary to know the nature of the light that corresponds to each color sense. As you know, light is a part of electromagnetic waves, and our eyes can sense the range between 380nm and 780nm. As you know well with the rainbow colors, the relationship of the wavelength of light and colors is as shown in Figure 7 below.*

Though characteristics of radiated light differ by the types of light sources, such as sunlight, light of mercury lamp, and electric light, each contain light of the range almost between 380nm and 780nm. Characteristics of light coming into eyes are known by the way of reflection of light by the object.
For example, as red paper absorbs light of short wavelength and reflects well light of long wavelength, the light coming into eyes is little light with short wavelength and much light with long wavelength, therefore, the paper looks red to our eyes. On the other hand, green paper absorbs both light of short and long wavelength, much light with middle wavelength comes into our eyes, therefore the paper looks green.

[Fig.7 : Spectrum]