Subtractive mixture occurs whenever substances which reflect light are combined, as in all painter's colors. It can be understood in this way: imagine that colorless light is a whole made up of three thirds. Every patch colored with nontransparent, primary color reflects one third and absorbs two thirds. If we mix these patches of red, blue, and yellow together, then three times one third is reflected, and three times two thirds absorbed. Only one third of the whole lighf energy encountering the surface is reflected, and two thirds is absorbed; the combination of reflected light is no longer white but darker, which means gray.
In the color circle this subtractive mixture of equal parts of primary colors can be represented as a concentric section of the circle which is dark gray. If you take this section eccentrically from the original circle we produce not a neutral, but a colored gray. The color corresponds to the one or two dominant colors. In the illustration it is a brown, since red + yellow = orange predominate. Otherwise expressed you have % red +% yellow
+% blue=% brown.
Subtractive mixtures can be made visible by the following experiment: paint a color circle (in three or more segments, it makes no difference) onto a disc of cardboard and rotate it quickly. The eye cannot separate the different color stimuii and they are mixed optically to gray. If instead of the complete color circle you paint an eccentric section 011 the disc, the result, when it is rotated, is a colored gray: blue-gray, green-gray, yellowgray (beige or brown), and so on. All dull colors are made in this way, depending entirely on what section of the color circle, or what original colors are used. Gray would also result if only two colors were rotated, if they consisted of one primary color and a mixture of equal proportions of the two others, such as red and green (blue + yellow).
It can be seen that green lies directly opposite red on the circle. Every pair of colors lying diametrically opposite each other adds up to gray. These opposed colors are called complementary colors. They and their effects are decisively important in all color sensations, and therefore in painting. This is due to the fact that the nearer two colors approach complementary relations, the more the eye is stimulated. It tries to combine them as gray, but since it cannot do so each color seems brighter against its neighbor than it would if it stood alone in a neutral weak-colored surrounding. If you look at a snow scene in the twilight everything looks gray on gray. If you light a lamp the landscape immediately looks uniformly blue. The blue increases in strength the closer the lamp light is to orange, the complementary color to blue.
The same thing happens on a cloudy winter's day if direct sunlight suddenly falls on the gray-white landscape. The shadows turn to pure cobalt blue, even though the glistening snow shows no orange, but a pure white, because many more of the long light waves, red and yellow, penetrate the thick blanket of air between the landscape and the low lying winter sun. Where its direct rays do not fall, you feel all the more clearly the contrast, the weak blue reflection.
This does not occur only in winter light, of course, but everywhere and all the time; but it takes more observation and practice to notice it. This is what is meant in the art schools by the insistent exhortation to "see color in everything"; though often no further explanation is forthcoming for the innocent student.