In plain language
Color must never be the only way information is shown (errors, links, statuses, chart series).
Who it affects
People with color-vision deficiency, low vision, or constrained viewing conditions such as strong glare.
How to check
View the page in grayscale: can you still spot links, errors, required fields, selected items, and chart differences?. Worked example — A customer-support live-chat widget: view selected tabs/toggles/swatches in grayscale: if the selected state disappears without color, it fails.
Common misconception
Removing color entirely isn't the fix. The rule only requires that color not be the *only* way information is shown, a second cue like an icon or label alongside the color is enough. Real example: on a news outlet's article page, the alt text describes the image but leaves out information that is shown only through colors in the image (e.g., which line on the chart is which).