I've built a handful of small color tools over the past year, and the thing that keeps surprising me is how bad HSL is at the one job people reach for it to do: generating a set of colors that look evenly spaced.
If you've ever built a palette by walking the hue wheel in HSL, keeping saturation and lightness fixed and stepping the hue, you've probably seen the result. The yellows and greens come out glaring, the blues and purples look dark and muddy, and the whole set feels lopsided even though the numbers insist every color sits at "50% lightness."
That isn't your monitor or your eyes. It's HSL.
Why HSL lightness lies to you
HSL's L channel is a geometric construct, not a perceptual one. It's derived from the max and min of the RGB channels, which has almost nothing to do with how bright a color actually looks to a person. Yellow at L=50% looks far brighter than blue at L=50%, because human vision is much more sensitive to the green-ish wavelengths that dominate yellow than to blue.
