This is Mike Bostock's block 3014589 plus the CIE L*u*v* and XYZ color spaces from this plugin. Both L*a*b* and L*u*v* are intended as "uniform color spaces" -- matching perceptual color difference with euclidean distance. d3.hcl is L*a*b* in cylindrical (Hue-Chroma-Lightness) coordinates.