What is the significance of a change in color of lubricating grease during service?

In-service lubricating greases can change color due to thermal degradation, oxidation, or contamination.  The dyes used to color greases may lose color intensity when subjected to high temperatures.  Highly oxidized greases may become very dark to black.  Greases that are contaminated with water sometimes take on an emulsified or milky appearance.  When different greases are mixed in service, the resulting color may be somewhere between the colors of the individual greases.