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.
When Romeo Santos teams up with bachata legend Juan Luis Guerra, you know you are in for some wild romantic exaggerations. "Carmín" is all about total infatuation, comparing romance to high art. The singer feels his lover was made specifically for him ("Que ella es hecha para mí" [That she is made for me]) and looks at her like a fine masterpiece: "Con un dibujo de da vinci" (With a drawing by Da Vinci) and "Como de van gogh, acuarela a perfección" (Like a Van Gogh, watercolor to perfection). To stamp the romance, she leaves a kiss behind that stains his cheek bright crimson: "mancha mi mejilla de carmín" (stains my cheek with carmine).
Things get delightfully over-the-top when the lyrics describe what he would do to reach her. He claims he would cross the open sea riding a dolphin ("Cruzo alta mar en un delfín"), or push forward without sleep in a yola ("En una yola sin dormir"). In Dominican Spanish, a yola is a small, handcrafted wooden fishing boat, often associated with hazardous sea journeys. Claiming you will steer a tiny wooden rowboat through open waters just to see your muse is dramatically extra, but that fits the whole theme of a lover who inspires every melody and sigh ("Ella es la música, suspiros, armonía en mi canción" [She is the music, sighs, harmony in my song]).