One of my earliest color illusions, created back in the 1990s, when I was immersed in exploring the physiology of color perception.
Do the pleated sections of the fan really have two different shades of brown?
They don’t. Every pleated “accordion” section is exactly the same brown. Only the thin light and dark bands bordering each fold differ. Those subtle edge contrasts trick your visual system into seeing alternating tones where none exist.
The color is perfectly uniform, but your brain interprets it differently depending on the surrounding contrast—a classic example of simultaneous brightness contrast.