In just a few words, the speaker imagines sketching the whole winter in chalk. Chalk suggests pale, dusty lines on a dark surface, and the title, "Monochrome," fits that picture: a landscape drained of color, all grey and white and black. The speaker seems to feel that winter could be captured simply, in a single shade.
Then the last line interrupts: "but for a crow." The word "but" works like a small hitch. Something stops the plan. A crow is black, so it might seem to fit the monochrome scene, yet the poem suggests it somehow doesn't belong in a chalk drawing. Perhaps the crow is too alive, too sudden, or too sharply dark to be rendered. Judging by the phrasing, the speaker is not annoyed so much as quietly struck by it.
The mood is hushed, spare, and a little wry. The poem shows how a living detail can slip past any attempt to sum up a season.