http://www.w3.org/WAI/WCAG21/Techniques/failures/F81:

It would also not fail if the color chosen had sufficient luminosity difference (lightness) from the other text that it would be easily be seen as different if viewed in black and white. In these cases - the information would not be displayed in color (hue) alone but also in "presentation" or "lightness" respectively

What is "sufficient luminosity difference"? 3:1 as in G183? And if: Why do links with 3:1 contrast need further clues to be able to recognize them and required or error fields do not:

if additional visual confirmation is available when a user points or tabs to the link