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, 30 to 45 at display sizes only (a logo, a big headline); under 30, avoid it for text. The same measure picks black or white text everywhere on Chromary. The older WCAG 2 ratio and its grade (AAA, AA, AA large) are shown small under each for anyone working to that standard.
Use
AaMilk on ShaleLc 77 · passesWCAG 2 · 5.4 : 1 · AA
AaShale on MilkLc 73 · passesWCAG 2 · 5.4 : 1 · AA
Avoid
AaCharcoal on ShaleLc 23 · failsWCAG 2 · 3.1 : 1 · AA large
AaShale on CharcoalLc 21 · failsWCAG 2 · 3.1 : 1 · AA large