Nerve damage. The Tropical Fishlopaedia (every aquarist should have a copy) says this…
Abnormally dark coloration is a very common sign of stress but may also be seen during many other diseases…
Heightened or otherwise abnormal coloration may be a sign of damage to the central nervous system with resulting loss of control of the chromatophores. Possible causes are hypoxia, poisoning, acidosis / alkalosis, injury, bacterial infection, or tumour.
It can kill the fish but in most cases (ime) it’s either temporary or stays but does no harm. I’ve treated this before with a bactericide, which worked in a couple of cases