What does a transformer oil color chart reveal about oil condition?

A color chart provides a quick visual indicator of oil darkening, which can correlate with oxidation, contamination, or the presence of sludge and degradation byproducts. Lighter color generally suggests newer or cleaner oil, while darker oil can indicate aging, overheating, or dissolved contaminants. However color alone is not diagnostic. Some oils darken while still meeting key dielectric properties. Therefore color observations should be paired with lab tests such as acidity, interfacial tension, moisture, and DGA to confirm actual insulation health.