Saturation / Chroma

How much colour a colour has. Saturation is the distance from grey, and it is the property the nineteenth century suddenly acquired the ability to buy.

Where the word comes from

Albert Munsell separated chroma from hue and value in 1905 so that colour could be specified rather than described, giving each sample a coordinate in a solid. Strictly, chroma measures departure from a grey of the same lightness and saturation measures it relative to the sample's own brightness; in ordinary use the two words have collapsed into each other, and the distinction only bites when you are converting between colour spaces.

In the studio

Before industrial chemistry, high chroma was simply not available in most parts of the spectrum. A painter wanting an intense green had verdigris, which is unstable, or malachite, which is weak, or had to mix blue and yellow and accept the loss. Then, within a few decades, came chrome yellow, the cadmiums, cobalt blue, viridian, emerald green — brilliant, opaque and packaged in collapsible tin tubes. Impressionism is usually explained as a change in the way painters saw. It was at least as much a change in what the colourman had on the shelf.

Why it matters

Maximum chroma is not the same in every hue, and this is the fact that breaks most colour systems. Yellow reaches its most intense at a high lightness; blue and violet only reach theirs down in the dark. There is no such thing as a saturated pale blue — the pigment does not exist and neither does the screen. Any palette built as a grid of "the same saturation across every hue" is describing colours that cannot be made, and something has to give when it is rendered.

Today

The same limit governs displays: a colour specified outside the gamut is silently clipped to the nearest colour that can be shown, which is why a palette that looks correct in one browser goes flat in another. Perceptual spaces such as OKLCH at least make the failure visible, since they can tell you a requested chroma is impossible instead of quietly substituting something else.

Pigments used: Cadmium yellow · Viridian · Cobalt blue

Essays: How colour became a science · When colour arrived at the cinema

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