In the previous Chapter we investigated different mechanisms which produce color. We learned that
white light contains all frequencies (colors) in equal proportions, and that we could perceive color as an imbalance in the visible spectrum reaching our eyes.

In this chapter we want to concentrate on

Classification of Colors

Then we need to consider the mixing of colors.

One way to learn something about our perception of color is to start with lights for which we know both the intensity-distribution curves and the sensations that these lights produce, and then to ask what sensations are produced by combinations of these lights. In the section on

Color Mixing by Addition

we discuss combining two lights by shining them at the same spot on a white screen, a screen that diffusely reflects all visible wavelengths equally well. The light reflected to your eyes from that spot then contains both lights, it is an additive mixture. At every wavelength the intensities of the tow lights add, so the combined intensity distribution curve is the sum of the two individual curves.

If we shine white light on the screen, that portion of the screen looks white to us. If, instead, we reflect the white light off a silver object and onto the screen, the screen again looks white, not silver. Had we used a copper object instead of the silver, the light on the screen would look orange. A fluorescent red object would reflect light to the screen that looks no different than ordinary red light, and so on. Thus, by concentrating on these lights, rather than on the objects themselves, we reduce the gamut of colors. But we are still left with the pure colors of the spectrum, washed out colors, and a host of nonspectral colors, which have to be analyzed.

When one combines two colored lights, on gets an additive mixture , the combination of the two lights add. To make the original colored lights in the first place, one might shine two beams of white light through colored filters. But what happens, if one combines the filters themselves, that is places them back to back and shines one light through the combination on a white screen? Using a red and a green filter, one sees very little or nothing on the screen, one does not get the additive mixture. This is because a a filter does not add color to the light that shines through it, rather it transmits different amounts of wavelengths already contained in the incident light. A  red filter only transmits  red light. It absorbs blue and green light. The green filter transmits only green light, it absorbs red and blue light. In particular, the green filter absorbs any red light previously transmitted by the red filter. So by the time the light has passed through both filters, everything has been absorbed and nothing gets to the screen. Combining the filters in this way illustrates

Color Mixing by Subtraction

This process is called this way, because it is easiest to analyze the process involved by considering what each filter takes away from the original light beam.

The subtractive process is quite common. When we say an opaque object has a particular color, we generally mean that, when white light shines on it, the object absorbs (subtracts) certain wavelengths from the white light, reflecting the rest to our eyes. The color of the object is determined by the reflected part of the incident light, the part that the object does not absorb. Thus, if we know what is taken way from white light, we know the resulting color. For example, a robins's egg is blue, because the shell absorbs green and red light, reflecting only the blue.