Drinking-water should ideally have no visible color. Color in drinking-water is usually due to the presence of colored organic matter (primarily humic and fulvic acids) associated with the humus fraction of soil. Color is also strongly influenced by the presence of iron and other metals, either as natural impurities or as corrosion products.

It may also result from the contamination of the water source with industrial effluents and may be the first indication of a hazardous situation. The source of color in a drinking-water supply should be investigated, particularly if a substantial change has taken place.

Most people can detect color above 15 true color units (TCU) in a glass of water. Levels of color below 15 TCU are often acceptable to consumers. High color from natural organic carbon (e.g. humics) could also indicate a high propensity to produce by-products from disinfection processes. No health-based guideline value is proposed for color in drinking-water.