Iron nitrate is colorless or lavender monoclinic crystal,
True Fe3+ ions and hexahydrate Fe3+ ions are lavender.
However, due to the usual pH conditions, hydrated Fe3+ ions are easily hydrolyzed.
Partially hydrolyzed products such as Fe(OH)2+ and other hydroxylates and oxyhydroxides, OH- and O2- as ligands, their coordination field effect is very strong.
The 3d energy level of Fe3+ is changed to yellow and yellowish brown, which also makes the solution yellow.