Noun
(slang, figuratively)
A clue or indication (especially on an online dating profile) that a person is boring or unoriginal. [from 2022]
A mundane but mildly unusual quirk or habit of one's romantic partner. [from 2023]
(by extension) Someone who has a beige flag (in either sense).
That guy is a huge beige flag.
Used other than figuratively or idiomatically: see beige, flag.
Source: en.wiktionary.org