Color Contrast Analyser for Sketch
Update from @auxdesigner: Now it supports transparency
A Sketch plugin that calculates the color contrast of two layers and evaluates it against the WCAG. The test may pass AAA, AA or fail because of a lack of contrast. And even when you do not need to meet those requirements, you can get a feeling for the contrast when you get used to the values. This might help you design accessible content.
Update: Also works with single layers that are now checked against the artboard background color.
Note: This plugin works with solid text and fill colors. Transparency is not supported yet.
Color contrast of 4.5:1 or 3.0:1 for large text
Color contrast of 7.0:1 or 4.5:1 for large text
(Large text means regular text larger than 18pt or bold text larger than 14pt)
Read more about the Web Content Accessibility Guidelines 2.0 http://www.w3.org/WAI/WCAG20/quickref/#qr-visual-audio-contrast-contrast
Based on the algorithms described here: http://gmazzocato.altervista.org/colorwheel/algo.php