Red fermented tofu gets its color from red yeast rice.
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culture
A pungent, salty fermented tofu preserved in brine or rice wine. Often compared to blue cheese for its funky aroma. Also commonly written 腐乳 (fǔrǔ). Iconic brands include 王致和 (Beijing, since 1669) and 广合 (Guangdong).
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In everyday speech most people say 腐乳 (fǔrǔ); 豆腐乳 is the fuller, more formal form seen on labels and menus.