Why Ammonium Sulfate Is Not White Some Time?

The colour of normal ammonium sulfate can range from white to beige. Sometimes it will change colour when exposed to oxygen and water vapour in the air. Probably due to

Blue: Ammonium Cupric Sulfate, Basic Copper Sulfate
Black: ferrous oxide, copper oxide, etc.
Yellow: ferric ion, ammonium sulfide
Light green: ferrous ions, ferrous sulfate
Light blue-green: ammonium ferrous sulfate
Yellow-green: ferric ions in solution
Reddish-brown: ferrous ions, cuprous oxide
Red: Iron ions are mixed into the finished product. The oil removal effect before entering the ammonia distillation tower is not good, and the remaining ammonia water contains certain phenolic substances (brought in by the ammonia distillation section), which are white or light yellow when they come out, and turn red after a period of time