Button hierarchy carried by colour alone
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Rule · bucket: judgment · severity: note · detector: none · since: v0.1.0
Section: Things to avoid
Group: States and interaction

Forbidden:
  A blue primary beside a green secondary, identical in every other respect

Instead:
  Weights differ in **structure** — solid fill, outline, underlined text — not just hue. Two buttons differing only in colour are the same button to a colour-blind user, and if their contrast against each other is under 3:1 they are the same button to a low-vision user too.

This also means a button's fill or border is not decorative: it is the thing identifying the element as a button, so it carries the 3:1 non-text floor.