#667eea

Most accent colors break in one mode — too bright on white, or too dark on black. #667eea lands in the narrow luminance window (0.10–0.30) where a color clears 3:1 contrast against both extremes simultaneously. But dozens of colors hit that window. What makes this one feel magical rather than merely correct?

The answer is the gap between how bright it looks and how bright it actually is. At 66% HSL lightness, your eye expects a color too pale for white backgrounds. It isn't. The WCAG luminance formula weights blue at just 0.0722 — versus 0.7152 for green. #667eea has a very high blue value (234/255) that barely registers on the brightness scale, leaving a true relative luminance of 0.237. The color looks vivid. The math says it's mid-dark. Both are right.

Warm hues can technically hit the same window, but at their required luminance they look like what they are — dark burnt oranges, olive greens, muted reds. There's no surprise. The surprise is the whole trick. The cool half of the spectrum — sky through purple — shares this property: each hue can carry high chroma while keeping luminance low, so they look vivid at a depth that warm hues can't achieve without looking muddy. Any of them could have been the discovered color.