Prussian blue is formed when:
Ferrous sulphate reacts with FeCl3
Ferric sulphate reacts with K4Fe(CN)6
Ferrous ammonium sulphate reacts with FeCl3
Ammonium sulphate reacts with FeCl3
2Fe2SO43+3K4Fe(CN)6⟶Fe4Fe(CN)63+6K2SO4 Prussian blue