Why freshly prepared FeSO4 subjected NO2 gives brown black colour?

When freshly prepared FeSO4 is subjected to NO2, a reaction occurs where the iron(II) ions in FeSO4 are oxidized to iron(III) ions. This oxidation reaction results in the formation of a complex between iron(III) ions and NO2, which is responsible for the brown-black color observed. The complex formed absorbs certain wavelengths of light, leading to the perception of the brown-black color.