Slate, and exactly one signal

Four semantic roles. Amber means one thing only — a gate refused something — and nothing else in the interface is ever amber, so a refusal is the only thing on screen that glows.

Colour never carries the verdict alone

Roughly 8% of men cannot reliably separate red from green, and the verdict is precisely the signal that must not be missed. Glyph, layer name and indentation carry it; amber only reinforces. In the real terminal these map to ANSI colours rather than fixed hex, so the palette inherits whatever theme the user already runs.