Crimson is a colour, but in this poem it's also a feeling that can't be pinned down. The speaker starts with a plain statement, "Crimson is blood," then immediately pulls in two directions at once: crimson is "lovely" and "deadly," "different" and, by the end, "beautiful."
The middle lines hand the colour to each of the senses, and almost every one is unpleasant. It tastes like cold metal, smells like rotting flesh, sounds like nails on a chalkboard, and feels like ice on your eyeball. These images are sharp and slightly sickening, yet the speaker still calls the colour beautiful. That clash is the heart of the poem. The speaker seems fascinated rather than disgusted, drawn to something dangerous.
The mood is eerie and uncomfortable, but also calm. The short, repeated lines sound almost like a chant, and the flat tone makes the grisly details feel stranger. Returning to "blood" and "beautiful" at the close suggests the speaker has accepted that beauty and horror can share the same colour.