Two short lines open a love poem that is really about rescue. The speaker says someone fills the emptiness of their days and sings lullabies to their sleepless nights. So the loneliness is constant, day and night, and this person is the one thing that eases it. The word "lullabies" makes the presence feel gentle and protective, almost like being looked after as a child.
The closing couplet does the most work. Blue is the color of sadness, and the speaker says everything around them is blue. Against that, the loved one is "the only purple," a color that stands out, and one that sits close to blue while still being something different. The speaker doesn't say the person erases the sadness, only that they glow within it. The mood is tender and quietly grateful, with a little sadness lingering underneath. It's a small, simple poem, but the single image of one color against another says a lot about how much one person can mean.