Function quantize 

pub fn quantize(c: Rgb, palette: &[Rgb]) -> usize

Index of the entry in palette that looks most like c.

OKLab distance rather than distance in sRGB, for the same reason mix interpolates there: sRGB’s numbers are not spaced the way seeing is, so a nearest match computed in it picks visibly wrong entries in the mid tones.

Panics

If palette is empty.