A man (possibly the king of a realm) lives in a kingdom, its quite likely grey, and he discovers various colours in his castle / dungeon. You see a brilliant patch of colour in an otherwise totally grey scene as he discovers each colour. He rushes outside and paints the entire land colours, but this makes the inhabitants sick / depressed / angry etc. based on the characteristics expected of that color. Colors include (from memory) pink, cyan and yellow.
I first had this read to me in the late 70’s and as it was from my public library so may have been published considerably earlier, perhaps 1950’s or 1960’s.