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
AaChalk on CementLc 56 · large textWCAG 2 · 2.9 : 1 · fails
AaCement on ChalkLc 52 · large textWCAG 2 · 2.9 : 1 · fails
Avoid
AaCharcoal on CementLc 44 · display sizesWCAG 2 · 5.9 : 1 · AA
AaCement on CharcoalLc 41 · display sizesWCAG 2 · 5.9 : 1 · AA