# Chroma and saturation: understanding C* in paint samples

> Distinguish chroma from saturation, calculate C* from a* and b*, and compare color strength without confusing it with lightness or hue.

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PHOENiX Sample

PHOENiX Sample · 4 min read · Last updated 13 November 2026

A paint sample can belong to the same color family as another yet look more muted or more vivid. Calling one sample stronger is useful conversationally, but a production brief needs a more precise question: has its chroma changed, its lightness changed, or its hue shifted? This article isolates chroma so that the answer can be checked.

## Chroma and saturation are different attributes

CIE distinguishes saturation, judged relative to the area's own brightness, from chroma, judged relative to a similarly illuminated neutral reference. Everyday language often mixes them. Here, C* means CIELAB metric chroma, an approximate numerical correlate of perceived chroma; it is not a saturation percentage. [\[1\]](https://phoenixsample.com/insights-chroma-saturation.html#source-1) [\[2\]](https://phoenixsample.com/insights-chroma-saturation.html#source-2)

## Read C* as a distance, h as a direction

At a fixed L*, the a* and b* coordinates locate the sample in a plane. C*ab is its radial distance from the neutral axis. The hue angle hab describes the direction from the positive a* axis. Thus changing C* while retaining L* and h moves along one ray rather than toward a different hue. Use the full label C*ab when confusion with another color space is possible. [\[3\]](https://phoenixsample.com/insights-chroma-saturation.html#source-3)

Calculate C*ab = √(a*² + b*²). For the hue angle, use atan2(b*, a*) and express the result in degrees from 0° to less than 360°; a simple one-argument arctangent needs quadrant correction. C*ab has no universal maximum of 100. [\[3\]](https://phoenixsample.com/insights-chroma-saturation.html#source-3)

## A worked example at constant lightness

Consider two hypothetical coordinates at L* = 60. Sample A has a* = 6 and b* = 8; sample B has a* = 12 and b* = 16. Their chromas are 10 and 20, and both hue angles are approximately 53.13°. B has twice A's numerical C*ab, not a proven doubling of perceived vividness, pigment concentration or tinting strength. The example describes coordinates; it is not a coating recipe or a manufacturing tolerance.

![Original coordinate diagram: A and B lie on the same hue ray at the same L*. Their distances from the origin are C*ab = 10 and 20. The marks are schematic, not calibrated reproductions of paint colors. PHOENiX Sample.](https://phoenixsample.com/assets/images/science-v262/chroma.svg)

Original coordinate diagram: A and B lie on the same hue ray at the same L*. Their distances from the origin are C*ab = 10 and 20. The marks are schematic, not calibrated reproductions of paint colors. PHOENiX Sample.

| Sample | L* | a* | b* | C*ab | hab (°) |
| --- | --- | --- | --- | --- | --- |
| A | 60 | 6 | 8 | 10 | 53.13 |
| B | 60 | 12 | 16 | 20 | 53.13 |

## Why hue becomes unstable near neutral

At a* = b* = 0, C*ab is zero and the hue angle has no unique meaning. Close to that point, small coordinate changes can create large angular changes. For example, (0.1, 0.1) and (−0.1, 0.1) both have C*ab ≈ 0.141, but angles of 45° and 135°. Investigate the coordinates and measurement stability before describing this as a major visible hue change. This conclusion follows from the coordinate geometry, not from a visual experiment.

## Use chroma to write a clearer sample brief

When requesting a more muted version, identify the approved physical sample and ask for lower C*ab while retaining the intended L* and hue as closely as the material permits. Record what was actually achieved in all three coordinates. Review the resulting specimens together before approval. This separates a request for reduced chroma from a request for a lighter pastel or a different color family; those are different design decisions.

## What the number cannot tell you

C*ab alone does not identify a color: many lightness levels and hue directions can share the same chroma. It also does not specify pigment dosage or guarantee that a requested coordinate is achievable with the chosen coating and substrate. For sample development, treat a chroma target as one part of a physical specification, with material feasibility and visual approval still to be established.

## Practical conclusion

Use C*ab to describe how far a sample lies from the neutral axis in CIELAB, while retaining L* and hue information. Specify the intended change, check the physical specimens and record the resulting coordinates. Avoid treating chroma as a saturation percentage, a pigment recipe or a complete appearance judgment.

## References and further reading

1. [CIE — Chroma, e-ILV 17-22-074](https://cie.co.at/eilvterm/17-22-074)
2. [CIE — Saturation, e-ILV 17-22-073](https://cie.co.at/eilvterm/17-22-073)
3. [Konica Minolta — L*C*h color coordinates](https://www.konicaminolta.com/instruments/knowledge/color/part1/08.html)

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