"The blue dye indigo is one of the oldest s known to man, having been in use for more than 4000 years. Its preparation by complex extraction processes was described in Sanskrit writings, and it was used to dye Egyptian mummy cloth. When Julius Caesar invaded Britain in 55 B.C., he found that the local ' ("painted people") decorated themselves with woad, a form of indigo. And who, today, does not know the all-pervading blue jeans, frequently dyed with the same indigo, now manufactured synthetically. While the colors... can be adequately explained by , the all-encompassing is required to explain the color of organic molecules. These include dyes and pigments, both as they occur in nature as colorants in the animal and vegetable kingdoms from which they were first extracted, and also as the triumphs of the synthetic dye industry which have displaced natural dyes almost completely. ...[W]e use the term "colorant" to include all types of organic dyes and pigments involving electrons on more than one atom in organic molecules, while multi-atom color-producing systems involving charge transfer are deferred ..."