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.
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.
The expression echoes the classical Chinese poetic image 万緑叢中紅一点, “one red blossom amid boundless green,” associated with 王安石. It condenses the contrast into 万緑 “myriad greens” and 一紅 “one red.”