The speaker sees themselves as an artist who fills the world with color, but the people around them seem to want something dull and uniform. Worse, they don't notice the effort behind the work, or that the speaker's brush is already worn out. Painting here reads as a stand-in for the speaker's way of being: their warmth, creativity, and personality.
In the second half, the speaker tries to give in. They swap the brush and turn the rainbow to black and white, but it feels wrong. So they pour their own inner colors into the world to finish it. That's where it falls apart: the "kerosene-fueled paint" of others burns what the speaker offers, leaving only "smoke and ashes."
The mood is tired and quietly bitter, ending in loss. Judging by the imagery, the poem seems to be about giving too much of yourself to people who can't value it, and being burned out as a result.