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Warning signs, Prohibitory signs and Mandatory signs Warning Signs is a triangle which is black on yellow with a black border — warn of hazards. Prohibitor Signs is a red circle with oblique on white — prohibiting risky behavior. Mandatory Signs is a blue circle with white pictogram — actions required to safeguard health and avoid injury.” Spot the shape, know the action: a triangle warns, a red circle forbids and a blue circle instructs. Follow signs immediately.”