Simultaneous contrast
The same exact color looking lighter, darker, warmer, or cooler because your eye judges it against whatever surrounds it.
See it
What it is
Simultaneous contrast is the way a color appears to change because of the color around it. Put the same middle gray on black and white: it looks lighter on black and darker on white even though its pixels are identical. A colored surround can also push the center toward the surround's complement.
Michel Eugene Chevreul described the effect while studying complaints about colors in Gobelins tapestries, and Josef Albers later made it the engine of 'Interaction of Color.' Reach for it when a logo, swatch, skin tone, or neutral looks correct in the palette file but wrong in the actual layout. The neighbors may be changing the perception, not the value in the file.
Gotcha: an eyedropper cannot diagnose a perceptual comparison by itself. Sample the value to prove the pixels match, then judge it in context. When consistency matters, add a stable neutral buffer, adjust the surround, or make a context-specific optical correction instead of insisting that one hex must look identical everywhere.
Ask AI for it
Build a simultaneous-contrast demo with CSS Grid. Place two identical 96px squares using one --sample: #808080 custom property, one centered on #ffffff and one on #000000. Add a second pair using the same muted orange sample on a blue surround and a red surround. Display the computed background-color value under every center square to prove each pair is pixel-identical, and add a toggle that replaces all surrounds with the same neutral gray.