Colour does not replace the data sheet
Two greases of a similar colour can have different thickeners, base oils, additives and applications. Different colours do not necessarily mean different performance classes either. NLGI treats colour as an appearance characteristic, not as shorthand for operating properties.
Colour therefore helps you identify a product and describe a sample, but it should not be a selection criterion. On its own it tells you nothing reliable about operating temperature, speed, load, water resistance, shear behaviour or seal compatibility.
Where does grease colour come from?
Natural colour and dyes
The natural colour of a grease depends on its formulation; some products range from light brown to dark. A manufacturer may also add a dye to distinguish a product from others or make it easier to recognise during maintenance work. A blue, red, green or amber shade is not a universal code for temperature, load or thickener type.
Additives and solid lubricants
Graphite or molybdenum disulfide can give grease a dark or black appearance. That does not mean that every black grease contains one of these additives, or that either additive is needed in every bearing. Check the technical data sheet and application documentation for the specific product.
Colour after service
Grease can change appearance during service because of temperature, oxidation, contamination, water or mixing with another product. SKF notes that darkening may be associated with high temperature, oxidation, carbonisation or solid particles, but blackening does not automatically mean that the grease is no longer fit for use. Compare the sample with unused grease and assess it together with operating conditions.
When does a colour change need investigation?
Not every change means failure. A clear difference from fresh grease should nevertheless trigger a context check:
- check the temperature at the bearing or lubrication point and how it has changed since the last relubrication;
- look for water, a milky emulsion, metal particles, lumps, an unusual odour or excessive oil separation;
- compare the product name, batch and relubrication date with the lubrication-point record;
- establish whether a different grease was used for top-up and, if so, whether the two products were confirmed as compatible;
- if the cause is still unclear, take a sample according to the plant procedure and have it assessed instead of deciding on colour alone.
Interpret colour together with temperature, vibration, leakage, noise, seal condition and operating history. Visual inspection of a sample does not by itself confirm either correct lubrication or bearing damage.
What should you check instead of colour?
When selecting a grease or changing product, check at least:
- the intended application and the type of lubricated component;
- the base oil and thickener;
- the NLGI consistency grade;
- the temperature range, speed and load intended for the application;
- water resistance, corrosion protection and behaviour in contaminated conditions;
- compatibility with materials, seals and the grease used previously;
- the quantity, application method and interval specified by the machine or bearing documentation.
NLGI 2, a colour or the words “high temperature” are single pieces of information, and none of them replaces the full specification. If information conflicts, the machine or bearing instructions and the data sheet for the specific product take precedence.
Colour and data in the Molyguard catalogue
In the Molyguard 2022 catalogue, colour is one column of the grease table, alongside thickener, base oil, NLGI grade and operating temperature. The table shows why colour alone is not enough. AERO 300 and GS ROLLER DR are both listed as dark brown. AERO 300 is a lithium complex grease with a semi-synthetic base oil, described for heavily loaded roller bearings at high temperature. GS ROLLER DR has a calcium-based thickener and a mineral base oil, and the catalogue recommends it for rolling mills and for bearings in dryers, steamers and stenters. A similar appearance therefore does not mean the same composition or application. The black greases GS PL and LMP 180 contain graphite and molybdenum disulfide respectively, the additives described above. Application descriptions for every grease are in the Molyguard lubricants range. None of them is a recommendation for a specific bearing without data on how that bearing operates.
Example: grease darkening in a bearing
If grease at a roller bearing gradually darkens, do not conclude that “black grease is better” or that it has “definitely burnt”. First compare temperature and running time with the product’s intended conditions, check possible contamination and the relubrication method, then assess the sample against fresh grease. Only this combination of evidence lets you decide whether to change the quantity, interval, procedure or product.
How to prepare a grease enquiry
When you send an enquiry about a grease from the TC Kolor oils and greases range, include the machine name and model, the bearing or lubrication-point designation, speed, load, temperature, seal type, contact with water or chemicals, the current product, the dispensing method and a description of the observed change. Record colour as one feature of the sample, alongside consistency, odour and contamination.
