Which is brighter? A girl rainbow or a boy rainbow?

 ANSWER: A boy rainbow, of course.

This morning I was looking up the Sino-Korean word for "India ink" (수묵 水墨) to confirm that the 수 portion of the word was the Chinese character for "water" when I came across the word 수무지개, which is defined as "the brighter arch of a twin rainbow." The Korean word for a "double rainbow" or "twin rainbow" is 쌍무지개. Since 수 means "male," 수무지개 literally means "male rainbow." So, what is the Korean word for "the fainter arch of a twin rainbow"? The answer is 암무지개 since 암 means "female."

So, the brighter more beautiful portion of a double rainbow is male, and the fainter less beautiful portion is female? Does that seem fair? To suggest that girls are not as bright and beautiful as boys? I don't think so.

Yes, male birds do tend to be bigger and have brighter colors than female birds but it still does not seem fair to refer to the less beautiful portion of a double rainbow as "female," given that girls are prettier than boys, unless the boy is in a Korean boy band. Using 수 and 암 to refer to rainbows seems sexist, for some reason.

By the way, notice in the first photo below that 수묵 지다 means "to get smudged," and 수묵 치다 means "to cover up" or "gloss over (a mistake)." So, when using the two idioms, be sure to pronounce 지다 and 치다 correctly to avoid confusion.

From Dong-A's Prime Korean-English Dictionary

From Dong-A's Prime Korean-English Dictionary