Colours begins with bright, violent shades (red, blood, crimson, scarlet) and slowly drains them away. Each stanza is just three short words or phrases, so the poem moves in small, clipped steps, almost like a list being ticked off. The imagery shifts from blood to ash, then from fire's attack to skin, cells and bone, suggesting the speaker is thinking about the human body as something that burns, fades and wears down. The phrase "our given coat" makes skin sound like clothing handed to us, something we wear only for a while.
The mood is quiet, bleak and a little eerie. The "vampiric" touch adds a dark edge, but the ending is calmer than the opening: "Darken, / Leaving, / Greying." Nothing dramatic happens. The colour simply goes. The speaker seems to see life as a passage from vivid intensity to dull ash, and presents it without panic, in a cool, observing tone.