To evaluate text's color contrast, Lighthouse uses success criterion 1.4.3 from WCAG 2.1: Lighthouse flags text whose background and foreground colors don't have a sufficiently high contrast ratio:
How the Lighthouse color contrast audit fails # For example, if you've ever read something on your mobile device outside, you've probably experienced the need for text with sufficient contrast. While this issue is particularly challenging for people with low vision, low-contrast text can negatively affect the reading experience for all your users. For example, presenting light gray text on a white background makes it difficult for users to distinguish the shapes of the characters, which can reduce reading comprehension and slow down reading speed.
Text that has a low contrast ratio-that is, text whose brightness is too close to the background brightness-can be hard to read.
