Whiteness and yellowness indices reduce a near-white color response to one number. They can be useful for white cement, grout, gypsum, ceramics, sealants, panels and architectural coatings, but only when the named equation and measurement conditions are appropriate to the material.
What whiteness and yellowness indices mean
Whiteness generally increases as a suitable near-white material appears lighter and may include a bluish contribution, depending on the equation. Yellowness quantifies a shift from colorless or preferred white toward yellow for materials within the method’s scope. Neither index is a complete color description; retain spectral data and L*, a*, b* values.
Where the indices are useful
| Application | Method control |
|---|---|
| White cement, grout or gypsum | Fix mix, water, mould, cure, age, moisture, surface preparation and compaction. |
| White ceramic or sanitaryware | Fix body, glaze, firing, location, curvature and gloss condition. |
| Sealant, coating or panel | Fix substrate, film build, cure, thickness, opacity, texture and backing. |
| Ageing or weathering | Measure matched areas before and after exposure with conditioning and orientation controlled. |
Control the equation and measurement inputs
Name the whiteness or yellowness equation, illuminant, observer, geometry, aperture, specular mode and UV condition. ASTM E313 states that specimens compared should be the same material and have similar general appearance, including gloss, texture and—where not opaque—thickness and translucency. It also defines applicability limits for the colors covered by its indices.
Moisture, cure and surface roughness can move the index substantially. Fluorescent brighteners or UV-responsive coatings require a physical UV condition; a software illuminant change alone is not enough.
Interpret batches and ageing
Use the index with L*, a*, b* and spectral direction. A lower whiteness may come from lower lightness, increased yellowness, loss of blue fluorescence or a surface-finish change. For weathering, report the initial state, exposure, cleaning and post-exposure conditioning. Do not mix measurements from different equations or instrument conditions.
Avoid common errors
- Do not compare unlike materials because both are called white.
- Do not use a one-number index for saturated or tinted products outside its intended range.
- Do not compare damp cementitious material with a dry standard.
- Do not treat whiteness as opacity; a translucent sample can be backing-sensitive.
- Do not transfer a limit from one formula or illuminant to another.
Keep the visual reference
One-number indices support QC, but project approval should still use representative physical standards under the specified viewing conditions.
References and further reading
- ASTM E313-20(2025): yellowness and whiteness indices.
- Whiteness Index Explained.
- Yellowness Index Explained.
Continue with ceramic color and instrument selection.
Last reviewed 22/09/2026 · Level: Intermediate
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