Contrast
A macOS app for quick access to WCAG color contrast ratios → https://itunes.apple.com/de/app/contrast-color-accessibility/id1254981365?l=en&mt=12
Accessible to some - Manuel Matuzović
The Myths of Color Contrast Accessibility
UX Optimizations For Keyboard-Only And Assistive Technology Users — Smashing Magazine
WCAG 2.1 Primer
WCAG 2.1 is the standard used by EN 301 549, the European procurement rules, and you need to pass these success criteria to comply with the UK Public Sector Bodies (Websites and Mobile Applications) Accessibility Regulations 2018
ColorSafe
Empowering designers with beautiful and accessible color palettes based on WCAG Guidelines of text and background contrast ratios.