Blood released from capillaries and trapped under the skin and breakdown products of hemoglobin in the blood are chiefly responsible for the coloration of black eyes and other bruises.
In the case of black eyes, old blood is never really black, but dark purple and green.
The color of the pooled blood is magnified by the loose and transparent skin around the eyes, making a bruise there darker than it is on other parts of the body.
The chemicals mainly responsible for the changing colors of bruises are a series of products of the breakdown of hemoglobin, the oxygen-carrying compound in red blood cells.
An important one is biliverdin, which is green. There may also be bilirubin, which is yellow-brown.
The timing of the breakdown and the mixing of colors are not fully predictable, but at first bruises are usually dark blue, purple or crimson.
The color gradually changes to violet, green, dark yellow and pale yellow and finally disappears.
In one study pathologists found that they could conclude only that a yellow bruise was more than eighteen hours old.