The speaker describes feeling like a servant to pain and a slave to life itself. Nobody notices them, yet the pain never goes away. The "white butler" seems to be a stand-in for the speaker: someone who quietly serves, stays in the background, and is easy to overlook.
The last two lines carry the sharpest feeling. If the white butler vanished, no one would care, because nobody has white as a favourite colour. It's a strange, almost childlike reason, and that is what makes it sad. Being unwanted isn't even treated as a big deal; it comes down to a trivial preference. White is also plain and colourless, which fits the idea of someone blending into the background.
The mood is lonely and hopeless, but also quietly bitter. The short lines and simple wording make the speaker sound tired and worn down, as if they've stopped expecting anyone to notice.