"What does the color of grease have to do with its quality?"
Grease manufacturers use colorants simply to help facilitate the identification of greases and to make them more appealing, as opposed to just brown or black. Color can help users spot an incorrect grease if, for example, a certain color of grease emerges from a grease gun when another color of grease was expected.
The color of grease can also provide some indication of its overall quality. As grease degrades and becomes contaminated, it usually will begin to darken. This darkening, which can be more noticeable when compared to new grease, may be a sign that the grease has reached a condemning limit. Although it should not be surprising that grease darkens, the rate at which it darkens is the important factor.
Besides grease darkening due to operating and environmental conditions, a shift in color could be a sign that the grease has mixed with a different type of grease unintentionally. If this occurs, immediate action should be taken to determine how and why this has happened. Careless greasing with the wrong grease is more common than most people realize.
A quick solution would be to use different types of fittings for various types of grease, such as normal fittings for stand-alone bearings and button-head fittings for electric motor bearings. Another option would be to leave a dollop of grease remaining on the grease fitting to indicate the appropriate color. This should be done only when there is a single type of grease at the facility with that particular color. The grease type and color should also be marked clearly on the grease gun.
Remember, while the color of grease may have the potential to provide quality information, it is only intended as a branding to indicate the type of grease (usually by thickener type.) However, keep in mind that no grease color is guaranteed to specify a particular thickener type even within a single grease manufacturer’s offerings.
The table below shows how a particular grease color may refer to several different types of grease, especially among lithium-complex greases.