This page publishes my calculations of the hue purity of generic watercolor paint pigments, as explained in the section on hue purity and optimal colors.

This page includes CIELAB coordinates, averaged across all paint brands, for the pigments discussed in the guide to watercolor pigments.

The maximum hue purity or MacAdam limit was calculated from the CIE 1964 10° standard observer (Stiles & Burch colormatching functions) under an equal energy illuminant.

Each row lists, in order:

COLOR INDEX — generic C.I. name of the pigment (this is missing for convenience mixtures)
COLOR NAME — common pigment name or marketing name
L*,a*,b* — pigment location in the CIELAB color space
C — pigment CIELAB chroma, defined as (a*²+b*²)^0.5
hue angle — CIELAB hue angle, defined as arctan(b*/a*)
HP — CIELAB chroma of the maximum hue purity, given the hue angle and lightness L* of the pigment
C/HP — hue purity, ratio of the actual chroma to the maximum possible chroma for the given hue angle and lightness
chroma a*, chroma b* — a*b* coordinates for pigment chroma, rescaled so that chroma a*b* and hue purity x,y are equal for hansa yellow (PY97)
purity x, purity y — x,y coordinates for pigment hue purity; these do not correspond to any standard chromaticity space

The table lists pigments in ascending order of purity x, that is, left to right on an x,y plot of hue purity.

The highest hue purity, about 80%, is found in red orange or light red watercolor paints (such as pyrrol orange PO73, cadmium scarlet PR108 or pyrrol red PR254), and in ultramarine blue (PB29).