Argyria

Argyria is the condition that results from chronic exposure to, and thus poisoning by, silver or its salts; the obvious symptom is a strange but characteristic bluish-grey pallor of the skin and in its severest form, while rarely fatal, it can lead to more disturbing psychological manifestations. It may merely reveal underlying ones, one of which is synæsthesia (synesthesia).
All-in-all that is hardly a likely topic for a song.  Except, that is, when Esben & The Witch are involved...

Argyria
These strange metallic voices
Gleaming white and breeding light
Delve under your brow
Delve under my brow
Folding, unfolding
They whisper colours
Watch the rainbows flood
Dancing, we're dancing
Pebbles flirting with our toes
Listen to our heartbeats grow
Marble rivers whirl
Minerals swim like fishes
A glorious school of glittering
Innocent eyes, these innocent eyes
These strange metallic voices.