The colors of many natural objects and terrains were determined by visual matching with samples of known spectral reflectance (Munsell’s A Color Notation). There are only three important groups of colors. Foliage lies in a yellow-green region of dominant wave-length 557–574 mμ. Earths range between yellow and orange-red, 576–589 mμ. Water, sky, and distant objects are blue, 459–486 mμ. Colors of autumn foliage spread into the yellows and reds to the end of the spectrum. The outstanding characteristic of these terrain colors is their lack of saturation, most having an excitation purity of 40 percent or less. Furthermore, objects quickly lose saturation with increasing distance from the observer, so that even in the clearest weather encountered no object over away had an excitation purity of over 11 percent. To the color-deficient observer these colors appear even less saturated than to the normal. In addition, the yellow-to-green region of the spectrum, where the majority of discriminations must be made, is one in which color blinds show poor hue discrimination. Thus, in both hue and saturation the color deficient is at a decided disadvantage in discriminating terrains.
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