Fossil agglutinated foraminifera are composed of fine sedimentary grains that are glued together by organic cement. In sedimentary basins, fossils can be buried to depths of many kilometres with temperatures increasing towards metamorphism (>200oC) at depths of six or more kilometres.
As temperature increases with the depth of burial, the organic cement in foraminifera becomes darker and darker until carbonized black. The Foraminiferal Colour Index (FCI) was devised as an empirical tool to measure this colour change by comparison to a standard colour chart (Munsell colour system).
Colour change in microfossils is significant to the petroleum industry because it indicates burial temperatures that are necessary for the generation of hydrocarbons. Agglutinated foraminifera are sensitive to colour changes in the early stages of petroleum generation and therefore can be used to predicate the onset of petroleum generation. For the sake of categorization, five levels of thermal maturity are recognized as shown in the illustration below. The five stages are immature, premature, mature (oil window), late mature, and post-mature.
For more information on the thermal maturation of agglutinated foraminifera and FCI see the following references:
https://en.wikipedia.org/wiki/Foraminiferal_Colouration_Index
McNeil, Issler, and Snowdon (1996)
McNeil (1996
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McNeil, Schulze, Matys, and Bosak (2015)