WCAG 2.0 recommends using luminosity contrast ratio to check colour contrast
A little over three years ago I posted Luminosity contrast ratio, in which I mentioned that WCAG 2.0 recommends a different algorithm for calculating colour contrast than the one suggested by WCAG 1.0.
Since WCAG 2.0 is now a W3C Recommendation I think it’s in place with a reminder about this. Check that the tool you use for analysing colour contrast lets you choose the Luminosity Contrast Ratio algorithm. A couple that do are Gez Lemon’s Luminosity Colour Contrast Ratio Analyser and The Paciello Group’s Contrast Analyser.
Gez Lemon provides more details about the difference between the Luminosity Contrast Ratio algorithm and the old one (referred to as the AERT algorithm) in Luminosity Contrast Ratio Main Colour Contrast Analyser.