Geologic Map Colors:
In general, these colors apply only to sedimentary rocks, and plain colors apply only to marine rocks. Sedimentary rocks that form on land are depicted with these colors plus patterns, such as dots or stripes.
U.S. Geological Survey Standard
The other major rock classesigneous and metamorphichave their own color traditions, rather than standards. Reds and oranges are reserved for the igneous rocks: volcanics take orange while plutonics take red, and in both of these older rocks are shown with darker colors. Metamorphic rocks tend toward browns, olives, and other "complex" colors. All of them use patterns freely as well. This makes the production of geologic maps, whether for print or online publication, a skilled art.
These colors are those specified by the current standards of the U.S. Geological Survey, which tend to govern maps produced by the individual states and American scientists. Their history goes back to the 1880s (see older color lists). For the time spans associated with these units, see the Phanerozoic geologic time scale and the Precambrian geologic time scale. The International Commission on Stratigraphy maintained its own color standard for many years with a notably different esthetic, particularly in the Precambrian. In 2009 the Committee for the Geologic Map of the World issued a new color standard that split the difference between the two. All of my geologic time scales use that standard, but old maps reflect the standards of the nation and time that produced them.