Colour carries the whole poem. The speaker addresses someone who has left a permanent mark on them, and that mark is vermillion, a red that suggests blood, shame, and passion all at once. Stanza by stanza, the stain spreads: to clothes, lungs, skin, and finally memory. Each time, the speaker says it is "forever," so the refrain works like a sentence that can't be appealed.
The feeling is a tangled mix of pain and attachment. The speaker bleeds, screams, and tries to hide the mark, yet also "breathes in" the other person, which suggests they can't fully let go. Judging by the last stanza, the harm isn't entirely one-way: "we stain each other." Even bathing in white, an attempt at cleansing, can't wipe out the name.
The mood is intense and claustrophobic. The repeated lines and the insistent red give it the feel of an obsession that keeps circling back, with no clear way out.