When we have stated that the color is yellow or green or blue and that it is dark or light, we have indicated two of its important qualities - its Hue and its Value, but by no means have we described it completely. We may say that an emerald is green and that it is light, but we can say that certain grapes are green and also light. Yet there is a decided difference between their respective colors if we place them side by side. Both may be green and of the same Value of light, but the emerald is strong in color and the grape is weak in color or grayer. This difference is measured in the dimension of Chroma. The scale of Value may be referred to in the convenient and easily understood term of a vertical pole. The pole represents a neutral axis to all the circle of Hues and in itself has no color, but is pure gray. Around this pole we may place our band representing the scale of Hue. Then if we imagine any one of these Hues on the circumference of the band to grow inward toward the gray pole in the center, growing grayer or weaker in color strength until it reaches this center pole and loses its color entirely, we have grasped the idea of the dimension known as Chroma. By dividing this into regular measured steps, we have a scale upon which the strength of color may be measured. This dimension of Chroma is written in a color formula by means of a numeral below a line. The numeral denotes the step upon the Chroma color scale at which it falls, thus /5, /8, /9, etc.
All of the Hues may be measured on this dimension at right angles to the vertical pole and grading from gray, step by step away from the pole to greater and greater strength of color.
Mr. Munsell, in his book "Color Notation," refers to "The Color Sphere." * This is a general form which enables us to consider color in an orderly fashion. Within this form all color can be balanced, as will be shown later. But in the actual measurement of pigment colors, such as we use in printing or painting, all Chroma color paths would not be of the same length nor would they all be fit within a sphere. Certain of them would extend to points outside of the sphere. Nor would all the Chroma color paths reach their greatest length at the equator of the sphere (the level of Middle Value). There are two reasons for this which are important to understand. First: By nature, colors differ in their Chroma strength, some being much more powerful than others. For example, The strongest red pigment used is twice as powerful as the strongest blue-green pigment and will require a correspondingly greater number of steps on a longer path to reach gray. The Chroma path of Red is the longest and extends far outside the sphere. It is 10 measured steps from the neutral pole.** Blue-Green is the shortest, being only five steps. The sphere is limited in size to this shortest axis for reasons which will appear when we take up the question of Balance or harmony of color. The second reason is: All colors do not reach their maximum Chroma Strength at the same level of Value. It can be readily understood, for example, that the strongest yellow pigment is by nature much lighter, or higher in Value, than the strongest blue pigment and, therefore, that the complete Chroma paths of these two colors will each touch the neutral pole at different levels.
Purple-Blue reaches its maximum Chroma at the fourth step of Value.
Yellow reaches its maximum Chroma at the seventh step of Value.
We have described each of the three dimensions by which any color may be measured ( Hue, Value and Chroma) and noted how each is written in a color formula.It only remains to put these separate notations together and to write a complete color formula encompassing all three dimensions. For example, we are given a certain color to measure and define and we find that
on the scale of Hue it is Red-Purple. When comparing it with the scale of Value, we find it is but three steps from the bottom and that it is only two steps away from the neutral gray pole on the scale of Chroma. A complete formula for this color would, therefore, be written R-P 3/2.
It is scarcely necessary to point out the practical advantages of such a system of definite measurement and notation over the vague and variable terms in general use, which are borrowed from the vegetable and animal kingdoms, such as olive, plum, fawn, mouse, etc., of which no two people ever have quite the same idea.
It is hoped that the foregoing explanation of the three dimensions of color has been sufficiently clear to convey to the reader a distinct mental image of what is meant by the terms Hue, Value and Chroma, in order that we may proceed to the study of certain principles of order for the intelligent and harmonious use of color (Complementary Colors, Balance and Color Combinations), which grow out of this simple and logical system of measurement.
* This is the Chroma of vermillion in dry pigment form. Red printing inks are now made (Ed. Note: When this was written in 1921) which are considerably stronger than ten steps of Chroma color.
** The Munsell Color Sphere is a globe, the north pole of which represents white, the south pole black, and the axis made up of a sequence of grays extending from white to black. Around the equator is a band of Hues whose Value is 5/ and Chroma /5. Above the equator are bands of Hues of successively higher Value, while below it are bands of successively lower Value. Upon rotating the sphere each band of Hues turns to gray. (Condensed from "A Color Notation," Munsell Color Co., Boston, 1919, page 19.)