Check a colour pair actually meets WCAG

Contrast is the accessibility requirement most likely to be broken by a decision nobody thought of as an accessibility decision. A brand colour gets chosen for a logo, then used for body text, and a palette that looked confident on a designer's screen becomes unreadable on a laptop in a bright room. The ratio is arithmetic, so it is one of the few parts of this you can settle rather than argue about.

The thresholds are worth knowing rather than looking up each time. Normal text needs 4.5 to 1 for AA. Large text, which means roughly 18 point or 14 point bold, needs 3 to 1. AAA raises those to 7 and 4.5. Most organisations aim at AA, and in the UK public sector accessibility regulations make AA the explicit target rather than an aspiration.

Two mistakes recur. The first is testing the colour you meant rather than the colour that renders: text at reduced opacity over a tinted background is a different pair from the one in the design file, and it is the rendered pair that people read. The second is forgetting non-text elements, because an icon carrying meaning, a form border or a focus ring has its own 3 to 1 requirement, and focus rings in particular are routinely styled into invisibility.

It is also worth being clear about what passing does not mean. A ratio is a floor, not a verdict. Text can clear 4.5 to 1 and still be hard work at nine pixels, in a light weight, over a photograph. The number catches the failures that are indefensible; judgement catches the rest.

The practical way to use this is early, on the palette, rather than late, on a page. Fixing a brand colour that fails at the point it is chosen costs a conversation. Fixing it after it has been applied across a site, a set of documents and an email template costs considerably more, and by then somebody has usually become attached to it.

For an organisation, this is where accessibility stops being a checklist and starts being a design system question: the tokens either pass or they do not, and if they pass, everything built from them inherits that. Getting it right once at the token level is the difference between accessible by default and accessible where somebody remembered.