1.6 The six attributes of color

When we describe a color we are describing three things at once: which hue it is, how light it is, and how vivid it is. CIE names these carefully, and it names six attributes rather than three, because there are different ways to describe how light and how strong the color is. Brightness is how much light a thing seems to give off; lightness is its brightness compared with a white in the same light. Colorfulness is how much color a thing seems to have; chroma is its colorfulness compared with a white in the same light. Saturation is colorfulness compared with the thing’s own brightness. Hue is the one attribute with no partner. The distinction matters because a red car is the same red in sunlight and in shade: its brightness and its colorfulness fall in the shade, its lightness, chroma and hue do not, and lightness, chroma and hue are what we mean when we say what color a thing is. The relative versions hold steady because of adaptation (Section 1.5): the visual system rescales its signals to the light it is in, so the ratio of a surface’s signal to the white’s stays nearly the same as the light gets brighter or dimmer, and that ratio is what lightness and chroma report. A paint has a lightness, a chroma and a hue; its brightness and colorfulness depend on the light it is seen in. Saturation is a property of both light and surfaces. It compares a color only with its own brightness, so it needs no white beside it, and it applies as well to a light seen alone in the dark as to a paint. The CIE’s exact wording for all six is collected in the math box at the end of this lesson.

CIELAB, the next lesson, provides only hue, value and chroma, which is all a surface color needs; when saturation is wanted, chroma divided by lightness is used. Painting manuals use saturation, intensity and chroma as one word. Of the three, chroma is nearly always what is meant, since a painter judging a paint on the palette is comparing it with the white beside it. The rest of these notes use chroma, lightness or value, hue, and saturation in the CIE’s senses. Where it calls a paint vivid, in the everyday sense, any number given is its chroma.

The six attributes. The CIE’s own definitions are in its vocabulary document [13], and Fairchild’s book sets them out with the experiments behind them [23]. Brightness is the attribute of a visual perception according to which an area appears to emit more or less light. Lightness or Value is the brightness of an area judged relative to the brightness of a similarly illuminated area that appears white. Colorfulness is the attribute according to which the perceived color of an area appears to be more or less chromatic. Chroma is the colorfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears white. Saturation is the colorfulness of an area judged in proportion to its own brightness. Hue is the attribute according to which an area appears similar to one of the perceived colors red, yellow, green and blue, or to a combination of two of them. So there are two absolute attributes, brightness and colorfulness, and their relative versions, value and chroma, each obtained by dividing by the white; saturation, which is colorfulness divided by the color’s own brightness, or equivalently chroma divided by lightness; and hue.

Sources

[13] CIE S 017:2020, ILV: International Lighting Vocabulary, 2nd ed., Commission Internationale de l’Éclairage, Vienna, 2020.

[23] M. D. Fairchild, Color Appearance Models, 3rd ed., Wiley, 2013.

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