Traditional hex codes (#3B82F6) and RGB values are hardware-oriented color spaces: they describe how computer monitor sub-pixels emit light, not how human eyes perceive brightness.
In standard HSL, a pure yellow with Lightness 50% (hsl(60, 100%, 50%)) looks blindingly bright, while a pure blue with Lightness 50% (hsl(240, 100%, 50%)) looks dark. This makes automated accessibility contrast impossible to predict.
Modern design systems build palettes using perceptually uniform color spaces like OKLCH or CIELAB:
0.7 has the exact same perceived brightness whether the hue is yellow, blue, green, or red.blue-600 and red-600 have identical contrast ratios against white).Level 3 · Design Tokens & Foundations