ContrastGraded by APCA, the Accessible Perceptual Contrast Algorithm, which measures how strongly text stands off its ground the way eyes see it, as lightness contrast (Lc) from 0 to about 106. Lc 60 and up reads as body text, 45 to 60 as large text and headlines; under 45, avoid it for text. The same measure picks black or white text everywhere on Chromary. The older WCAG 2 ratio is shown small under each for anyone quoting that standard.
Use
AaChalk on OchreLc 57 · large textWCAG 2 · 3.0 : 1
AaOchre on ChalkLc 53 · large textWCAG 2 · 3.0 : 1