This is the second dimension of color and is possibly the simplest to understand. It is, according to Albert Munsell's definition, "The quality by which we distinguish a light color from a dark one." We noted that the first dimension did not tell us whether a color was light or dark.
It told us that the color was green and not red, but we know that there may be light green and dark green. The function of Value is to tell us how light or how dark a given color is.
For this purpose we shall need a scale of Value, which we may conceive as a vertical pole, or axis, to our circle of Hues. Black is at the lower end, representing total absence of light. White is at the top, representing pure light. Between these are a number of divisions of gray, regularly graded between black and white. This gradation could also be infinite. Since pure black is unattainable, we will call that 0 and begin our scale with the darkest gray as 1, numbering the steps up to 9, which is the lightest gray. Pure white, which is also unattainable, we will call 10. In the practical use of the scale of Value, therefore, we shall have but 9 steps and the middle one of these will be 5 - what is referred to as Middle Value. These steps of Value, have been scientifically measured and registered by means of an instrument called a Photometer.* In writing a color formula we express this dimension of Value by a numeral, which denotes at what step upon the scale of Value this color falls. This numeral is written above a line, as B 6/ for example, by which we mean that this particular blue, regardless of its other qualities, is as light or as dark as the 6th step upon the scale of Value. A color such as is commonly called "maroon" is an example of a red which is low in Value, because it is dark. What is called "pink" is a red which is high in Value because it is light. Now having familiarized ourselves with these two dimensions and understanding what qualities of a color they express, we may proceed to consider the third dimension. Without this third dimension our description of any given color is incomplete.
*This instrument is described in Albert Munsell's book, "A Color Notation." Munsell Color Co., Boston, 1919, p. 38.