Color is not a fixed property of objects but a perception shaped by context. The phenomenon of simultaneous contrast, famously studied by French chemist Michel Eugène Chevreul in his 1839 work The Law of Simultaneous Contrast of Colors, demonstrates that a color's appearance changes depending on the colors around it — a neutral gray looks warmer on a blue background and cooler on an orange one. Closely related is the Bezold effect, named for Wilhelm von Bezold, which shows that introducing new light or dark colors into a composition can shift the perceived temperature of the whole. These effects reveal that color is inherently relational.
Several other perceptual phenomena illuminate how our visual system processes color. Color constancy allows us to perceive an object's color as consistent despite changes in illumination, so a white shirt looks white under candlelight, fluorescent light, or daylight. The mechanism behind this, chromatic adaptation, was at the heart of the viral 'The Dress' debate, where viewers' brains adjusted differently to ambiguous lighting cues. An afterimage occurs when prolonged staring at one color fatigues specific cone cells, causing the complementary color to appear when the gaze shifts to a neutral surface. Color blindness, or color vision deficiency, demonstrates what happens when those cones are missing or malfunctioning — most commonly affecting red-green discrimination in roughly 8 percent of men and 0.5 percent of women.
Different forms of color blindness arise from different cone deficiencies. Protanopia results from absent red cones, making reds appear darker or black. Deuteranopia, the most common form, arises from missing green cones, causing confusion between reds and greens. Tritanopia, a rare type involving missing blue cones, makes it hard to distinguish blue from green and yellow from violet. Designers accommodate these variations by never relying on color alone, pairing color with icons, patterns, or labels, ensuring sufficient contrast, and testing with simulators such as Color Oracle or Coblis.