A status effect, probably caused by a potion, that removes all color from the game, but affecting more than just the map: color-based items cannot be differentiated, color-based monsters will not be distinguishable (e.g. a red naga and a black naga will just show up as “naga”, all dragons will just be “dragon”), and rays are reduced to some sort of generic ray effect (all dragon breath is just generic dragonbreath with no particular type, similar for all wand zaps).