Music, in this short poem, is a kind of switch that changes how the speaker sees everything. Once the listening starts, the world looks brighter, colors seem richer, and even people feel nicer. The speaker doesn't say the world has actually changed. Instead, the poem suggests that music colors perception, so ordinary things look warmer and kinder.
The ending is where the feeling settles. The last four lines slow down, and the words "Simply / Fade / Away" are each given their own line, so the poem seems to dissolve the way the speaker's thoughts do. That is the real payoff: music doesn't only make things prettier, it also gives relief from overthinking. The mood is calm, gentle, and a little dreamy. The language is plain and the lines are short, which fits the idea of a mind growing lighter and quieter. It reads like a small sigh of relief, the kind of moment many of us know when we put on headphones and let everything else drop away.