From the US Standards Website::

“WCAG (Web Content Accessibility Guidelines) ensure that content is accessible by everyone, regardless of disability or user device. To meet these standards, text and interactive elements should have a color contrast ratio of at least 4.5:1. This ensures that viewers who cannot see the full color spectrum are able to read the text.”

“The options below offer color palette combinations that fall within the range of Section 508 compliant foreground/background color contrast ratios.”