빨간 and 붉은 both mean the color “red” as an adjective. They can mostly be used interchangeably, but there are situations where one is more usable. 붉은 is used when something is naturally that color, for example, your lips are red, so you would say 붉은 입술 (bulg-eun ib-sul). However, if you wanted to describe an object as red, for example, a cup, you would say 빨간 컵 (bal-gan-keob). That specific cup is red, but not all cups are red.
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