The blue colour of copper sulphate changes to green colour on reacting with iron.
The reaction between copper and iron
When an iron nail dipped in the solution of copper sulphate then iron displaces copper from the solution of copper sulphate because iron is more reactive than copper. Therefore copper sulphate solution colour changes from blue to pale green.
CuSO4 (aq) + Fe (s) → FeSO4 (aq) + Cu (s)
When an iron nail is dipped in copper sulphate solution, a brown coating of copper is created on the surface of iron and the colour of the copper sulphate solution turns from blue to light green. This reaction explains that iron is more reactive than copper as it displaces copper from its solution and iron passes into solution as Fe2+ ions and ferrous sulphate solution is formed.