The above diagram, displaying a circle of the 10 regular Hues arranged in the immutable order imposed by the spectrum, reading clockwise beginning with Red at the top, will serve, with little explanation, to illustrate what is meant by "opposite," or the more familiar term "complementary," colors.
The term opposite is the preferred term in the Munsell Color System because it is simple and self-explanatory, as can be seen by reference to the above diagram, where each Hue on the circle will be found directly opposite another Hue.
Thus a straight line drawn from Red on the circle of Hues through the neutral pole will pass through Blue-Green, its opposite or complementary color. A line from Yellow through the neutral pole will pass through Purple-Blue and so on throughout the whole circle. It should be noted that each of the simple Hues (Red, Purple, Blue, Green and Yellow) falls opposite a compound Hue (Blue-Green, Green-Yellow, Yellow-Red, etc.). Now two colors which are opposite to each other are not only farthest apart upon the diagram, but are in actual use the most strongly contrasting. It does not matter at which point we draw the line, whether it is from one of the regular Hues or from a point between two regular Hues, if it passes through the center, it will fall upon the Hue or intermediary Hue which is its strongest contrast. This may be easier to visualize if we imagine the spindle indicated on the diagram as pivoted on the neutral pole and movable to any point on the circle.
It may be asked how it is determined that these colors which fall opposite to one another on the scale of Hue, are, in fact, the most strongly contrasting colors. The answer to this question will serve to demonstrate the logical foundation of the Munsell Color System. When any two colors are truly opposite (at the point of strongest contrast), their admixture* will produce a perfectly neutral gray. Though this may be accepted as indisputable, it can be easily proven with scientific accuracy by arranging two opposite colors on a disc in proportions relative to the Chroma strength of each and revolving them with such rapidity that we cannot see them separately and they become mixed. If they are truly opposite, they will unite in a perfect gray. Therefore, working back from this fact, the scale of Hue has been so composed that those colors which when mixed with each other do actually make gray, are placed directly opposite on a line running through the neutral gray pole.
Another question which may arise is - what will take place if we draw a straight line between two Hues which are not opposites and what would be the result of the admixture of them? This can best be answered by the diagram below, where three different lines have been drawn, none of them going through the neutral center. These lines, it will be seen at once, cross points which are not neutral, but are nearer to one or another of the Hues lying between the ones from which the lines are drawn. The result of the admixture obtained is seen in the diagram. This will be sufficient to further demonstrate the simplicity and logic of the Munsell Color System and to suggest to the reader other interesting examples of it.