Check a text and background pair against the WCAG 2.1 thresholds — 4.5:1 for body text, 3:1 for large text and for anything you have to be able to see — with the ratio, the verdict, and real sample text at three sizes.
When a pair fails, it offers the nearest colour that passes: same hue, same chroma, lightness moved only as far as it had to be. A translucent colour is composited over the background first, because that is what the eye actually judges.