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to become red; to redden; to blush
褒められて彼女の頬が少し赤らんだ。
Her cheeks turned slightly red when she was praised.
恥ずかしくて顔が赤らんでしまった。
I was so embarrassed that my face turned red.
夕日に照らされて空が赤らんでいる。
The sky is reddening, illuminated by the setting sun.