Iron red, iron yellow:

Ferrous hydroxide is produced by neutralizing ferrous sulfate with caustic soda (or ammonia). Ferrous sulfate is oxidized by air in the presence of crystal seeds. Ferrous sulfate and alkali (or ammonia) are added continuously in the reaction process to maintain a certain ferrous concentration and pH value. A series of hues of ferric oxide from light to dark can be obtained by controlling the progress of chromatic light. Iron yellow seed was obtained under acidic condition and iron red seed was obtained under alkaline condition. 

Ammonia iron red:

The primary seed γ - FeOOH was prepared by neutralizing ferrous sulfate to alkalinity (pH = 8.5-9.5) with ammonia and oxidizing with air. The transformation of γ - FeOOH into α - Fe2O3 under the induction of Fe2 + is essentially a re nucleation process. In the presence of α - Fe2O3 crystal nucleus and ferrous sulfate, ferrous sulfate is oxidized by air, and the resulting ferric oxide is deposited on the crystal nucleus. Meanwhile, the generated sulfuric acid is neutralized by ammonia, and ferrous sulfate and ammonia are added continuously. The crystal nucleus grows gradually to form iron red.

Ammonia iron yellow: 

The ferrous sulfate solution is neutralized with ammonia to pH 5-6. After neutralization, there is still a certain amount of ferrous sulfate in the system. The iron yellow seed is oxidized by air at room temperature. In the presence of seed, the two-step oxidation process is similar to that of ammonia iron red.