Carmín is a specific, deep red color, often translated as 'carmine' or 'crimson'. It's a much more poetic and artistic word than the common Spanish word for red, rojo.
In the song, Romeo Santos sings "Y mancha mi mejilla de carmín" (And stains my cheek with crimson). This line beautifully captures the image of a lipstick stain left by a kiss, a tangible and intimate mark of his lover's affection. It's a perfect example of the song's romantic and detailed imagery, making the title itself a central piece of the story.
Romeo Santos teams up with the legendary Juan Luis Guerra for "Carmín," a bachata that is just overflowing with poetic, almost over-the-top romantic declarations. The singer is completely swept away by a new love, one that feels like a fantasy. He says, "Me hundo en fantasías muy profundas" (I sink into very deep fantasies) and that it's "Fascinante es sentir / Que ella es hecha para mí" (Fascinating to feel / That she was made for me).
This isn't just any love; it's a source of massive artistic inspiration. He compares her and his feelings to masterpieces, name-dropping both da Vinci and van Gogh. She's his muse, and for her, he'd do anything, including absurdly heroic things like "Cruzo alta mar en un delfín" (I cross the high seas on a dolphin). The song's title comes from a beautiful line where he's so flooded with emotion that she "mancha mi mejilla de carmín" (stains my cheek with carmine), like a kiss leaving a mark.