Color is one of the first noticed and most obvious characteristics of a rock, but it is also one of the most difficult to interpret. With the exception of gray and black, which mostly results from partially decayed organic matter, most rock colors are the result of iron staining. Ferric iron (Fe+3) produces red, purple, and yellow colors (from minerals like hematite and limonite). Ferrous iron (Fe+2) produces greenish colors.
From Sedimentary Rocks
Some broad interpretations may be made from a rock's color. Red colors mean well oxygenated environments, such as river channels, some flood plains, and very shallow marine. Green colors mean an environment low in, or lacking, oxygen, often associated with marine environments. Dark gray to black colors mean anoxic conditions, which may mean deep water, but could also be a swamp environment. The conclusion is, environmental interpretations can only be made in relation to the other evidence present with the rock.
When and how these colors originate in sedimentary rocks has been in long debate but many of the colors are digenetic. Thus, environmental interpretations from color must always be viewed suspiciously. Use color with caution in making interpretations, and check its conclusions with independent evidence.