1 The viewing card
Painters judge a colour by looking at it through a hole in a grey card, so the colours around it can't influence them. Press and drag on the painting to lower the card. Let go, and the surroundings come back.
Why does grey turn blue?
Your visual system does not read colour pixel by pixel. To keep a lemon yellow under candlelight and daylight alike, it estimates the colour of the light from the whole scene and discounts it. Edwin Land's retinex experiments showed that a patch's perceived colour depends on its relationship to its surroundings, not just on the light reaching the eye from that patch. Painters had relied on the same effect, simultaneous contrast, long before Chevreul described it in 1839.
These studies are deliberately warm: flesh, ochre, vermilion and warm browns. Your eyes treat that warmth as a property of the light and adjust for it. After the adjustment, a truly neutral grey looks as if it leans away from orange, toward blue. The card removes the context, and with it the correction.
How strong the effect is varies between people, screens and rooms. On a phone at full brightness in a dark room it is usually strongest. Some people barely see it in one study and clearly in another.
2 Paint it yourself
Mix dabs from the four tubes. The mixing is modelled the way paint behaves, with light absorbed and scattered through the layer (Kubelka–Munk), not by averaging RGB numbers. Your patch is shown inside a surround, so you can paint with the illusion.
Your paint, on its own
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Estimated look beside this surround
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What naive RGB averaging would predict
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Notes and honesty
- The "Zorn palette" is a later label. Teachers call these four colours after the Swedish painter Anders Zorn (1860–1920). Emma Jansson's technical study for the Zorn Museum (Zorn's Studio, 2023) argues against the idea that he limited himself to them, and secondary accounts of that research report cobalt blue, ultramarine and viridian among his materials. Treat the four tubes as a teaching discipline, not a historical rule.
- No pixel in the four studies has more blue than red. Every warm colour is a modelled mix of the four pigments. Every fully covered pixel of the cloak, sea, irises and jug decoration has identical red, green and blue values. The checker in the source folder re-renders every study and verifies both claims.
- The pigment curves are illustrative. They are smooth approximations of the general shape of published reflectance spectra, not measurements of a particular paint. Real ivory black tints lean slightly cool, and the model keeps that faint lean, which you can see in the readouts.
- "Estimated look" is a simple heuristic. It adapts partway to the surround's mean colour (von Kries scaling). It is not a full colour-appearance model, and your eyes are the real judge.