Same Until the Lamp

In daylight these two squares are the same colour. They are not the same paint. Under a warm incandescent bulb, once your eyes have adjusted, they come apart. Colours that match under one light and not another are called metamers. A new pair appears every day.

Why they match, and why they stop matching

Your eye reduces all the light coming off a surface to three numbers, one per kind of cone. Two very different reflection curves can land on the same three numbers under daylight. Change the light and the curves are weighted differently, so the numbers move apart.

How this was made

Each day's two reflectance curves are smooth, physically possible (between 3% and 95% at every wavelength), and chosen by optimisation to match within CIEDE2000 0.5 under CIE illuminant D65 (daylight) while differing as much as possible under CIE illuminant A (a 2848 K tungsten bulb), with both squares inside the sRGB gamut under both lights. Colours use the CIE 1931 2° observer, and the bulb view uses CAT02 chromatic adaptation to show what you see once your eyes have adjusted. The first flash is the bulb before adaptation. A screen can only simulate this: it shows you the colours your eye would report, not the paints themselves. CIE colour-matching data from CVRL.