"The Fechner color effect is an illusion of color seen when looking at certain rapidly changing or moving black-and-white patterns. They are also called pattern induced flicker colors (PIFCs). Not everyone sees the same colors.
The effect is most commonly demonstrated with a device known as Benham's top. It can also be seen in stroboscopic lights when flashes are set at certain critical speeds. Rotating fan blades, particularly aluminium ones, can also demonstrate the effect; as the fan accelerates or decelerates, the colours appear, drift, change and disappear. The stable running speed of the fan does not (normally) produce colours, suggesting that it is not an interference effect with the frequency of the illumination flicker.
The effect was noted by Gustav Fechner and Hermann von Helmholtz and propagated to English-speakers through Charles Benham's invention of his top. The perceptual mechanism of Fechner color is not entirely understood."