noun
one red flower in a sea of green; one standout among many; one woman among many men
A yojijukugo (four-character idiom) used metaphorically: a single striking element (often a woman or a quality item) that stands out in a large, uniform group. The literal image is a single red flower in a vast green landscape.
パーティー会場では、彼女がまさに万緑一紅の存在だった。
At the party, she was truly the one red flower in a sea of green.
この作品は凡作ばかりの展覧会で万緑一紅の輝きを放っている。
This piece shines as the one standout in an exhibition full of mediocre works.
紅一点 also means 'one woman among many men' but is more common and casual. 万緑一紅 is more literary and emphasizes the contrast between the one and the many.
A yojijukugo derived from classical Chinese poetry, combining 万緑 (ten thousand greens) and 一紅 (one red). The exact origin is uncertain, but it follows the common four-character pattern of contrasting a single vivid element against a vast background.