Colour Contrast Checker

No. Contrast is one success criterion out of dozens. A page can clear every ratio on this tool and still be impossible to use with a keyboard, unreadable to a screen reader, or confusing to navigate. Contrast is simply the easiest failure to find and the cheapest to fix, which is why it gets so much attention.

This tool compares two flat colours, so it cannot read a background that changes behind the letters. Sample the lightest and the darkest point of the image where the text sits, and test your text colour against both. If either fails, the text fails. In practice you are usually better off adding a solid panel or a scrim behind the text rather than trying to find a colour that survives the whole image.

Usually not. The requirement applies to a colour pairing, not to a colour on its own, so the same brand colour can be fine on one background and fail on another. Keep it for large headings, buttons and graphics where the threshold is lower, and use a darker or lighter variant for body copy. The suggestions in the tool hold the hue and only move the lightness, so what you get back still reads as your brand.

24px and above at a normal weight, or 18.66px and above when bold. That is the point where WCAG drops the requirement from 4.5:1 to 3:1. It is worth measuring rather than assuming, because a heading that looks large in a design often lands under the threshold once it is built and scaled down for mobile.

AA is the working standard and the one nearly every organisation is measured against, including under the Public Sector Bodies Accessibility Regulations. AAA is the enhanced level and is not expected across a whole site. It is worth targeting on long reading passages and on services an older audience depends on.

A separate rule covers the parts of an interface that are not text: input borders, meaningful icons, focus indicators and the lines on a chart. These need 3:1 against what sits next to them. Very pale grey form borders are the most common failure we see, and they matter more than they look, because they are what tells someone where to type.