The word is used to describe two different things, and much confusion has thence arisen. From the point of view of physics, the color of a light is simply measured by its wave-length. However, from the point of view of everyday experience, color is simply a subjective phenomenon, i.e. our apprehension, by means of eyes and mind, of various wavelengths and combinations of wavelengths. White light can be resolved into a spectrum or series of wave-lengths, the longest of these producing the effect of red upon the eye, and the shortest that of violet. The spectrum is continuous from end to end; the eye divides it roughly into six distinct colors, red, orange, yellow, green, blue, and violet, though these blend into one another. The same color sensation can be produced by various mixtures of wave-lengths; the sensation of white light can be produced by mixing two colors, or rather two single wave-lengths of light from different parts of the spectrum. Thus red mixed with a suitable amount of green of a suitable wavelength will produce a sensation of white, and the same is true of yellow and violet. Such pairs of colors are called complementary.
Colored objects owe their color to the fact that they absorb from white light certain wave-lengths, and reflect or re-diffuse others, the latter producing the color sensation in our eyes. Thus a red object is one which absorbs all light falling on it except red, and if we examine it by light which does not contain red, as for instance, pure green light, it will appear black. However, objects possessing perfectly pure color, whether single or mixed, are very rare; the majority partially reflect all wave-lengths.
Thus there is a difference between mixing pigments and mixing colors. If red and green paints of complementary colors are mixed, the resulting color will be black since the mixture will absorb all light falling upon it. However, if a centrally pivoted disc is painted with these colors in segments of the correct relative size, and the disc spun so that, to the eye, the colors are added, the resulting color is white.
As far as the mixing of colors from pigments is concerned, red, yellow and blue are known as primary colors, since all other colors can be mixed from these three. Green, orange and purple are known as secondary colors, since they are each the result of the mixing of two primary colors. All these colors can be arranged in succession around a circle. By again mixing adjacent colors further intermediate hues are obtained as indicated. A color can be defined in terms of three characteristics: its hue, tone and intensity. Hue is that quality which distinguishes red from blue, blue from* green, etc. Intensity is a measure of the quantitative sensation of hue. A white or gray has zero intensity; a bright color has a high intensity. Tone is measured by the proportion of light which is reflected from the colored surface; it can be reduced by adding black to a pigment.
The terms tint, shade and grayed color are also used. Tints are full strength hues which have been reduced in intensity by the addition of white. Shades are full strength hues which have been reduced in tone by the addition of black. Grayed colors are hues to which both white and black have been added.