Contrast is a ratio, not a vibe
Below 4:1 is flagged. The flag is not a scan.
Relative luminance follows the WCAG definition. Contrast is (lighter + 0.05) / (darker + 0.05). Black #111111 on white is about 19:1. A yellow module on a white poster can fall under 4:1 and is listed as a risk.
The tool does not look at gloss, lamination, or a photo behind the code. A ratio over 4:1 can still fail in a hallway. A ratio under 4:1 is a reason to change the artwork before you print a thousand copies.
Set the two colors on checker and read the risk row. Then print test sheet and scan it with the phone you expect customers to use.