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
AaCharcoal on Periwinkle BlueLc 49 · large textWCAG 2 · 6.7 : 1 · AA
AaClub Soda on Periwinkle BlueLc 51 · large textWCAG 2 · 2.5 : 1 · fails
AaPeriwinkle Blue on CharcoalLc 47 · large textWCAG 2 · 6.7 : 1 · AA
AaPeriwinkle Blue on Club SodaLc 47 · large textWCAG 2 · 2.5 : 1 · fails