Two gray squares with identical reflectance appear different shades because one lies in cast shadow, showing perceived lightness does not track spectral reflectance.

Evidence

Adelson’s Checker-Shadow Illusion - The Illusions Index (The Illusions Index)

XREF: connects to: White is the most inferior of colors of morals of superiority. It follows that whiteness is the lack of characteristics that make a theme and give it its facets of colors. ; Spectral reflectance does not determine perceived color, since any achromatic paper can be made to appear any shade of white, gray, or black, the physics of reflectance diverging from color perception. — A Spectral Reflectance Doth Not a Color Make; Colors are not properties of objects but of perceptual processes, and one proposal identifies them with physical ‘spectral surface reflectance.’ — The Reality of Color Is Perception

Spectral reflectance does not determine perceived color, since any achromatic paper can be made to appear any shade of white, gray, or black, the physics of reflectance diverging from color… (similarity 0.91)
Colors are not properties of objects but of perceptual processes, and one proposal identifies them with physical ‘spectral surface reflectance.’ (similarity 0.90)
In visual perception a color is almost never seen as it physically is, making color the most relative medium in art. (similarity 0.84)
There is generally no fixed relation between spectral distribution and perceived color, though a surface does have a determinable color under fixed viewing conditions. (similarity 0.82)
Philosophers examine whether whiteness is a color at all, distinguishing the physics of spectral reflectance from the perception of color, a divide that determines whether white counts as ‘a… (similarity 0.79)