If heated in open air to a certain temperature, copper would undergo the reaction below.
CuO is a black solid so you would expect your copper turn black.
3. Iron is more “reactive” than copper. So it displaces Copper from Copper Sulphate solution.
Copper Solution is blue in colour. Iron Sulphate solution is Green.
Iron has blackish appearance, while Copper is reddish brown.
Reaction proceeds as
Fe + CuSO4 -FeSO4 + Cu
Due Iron Sulphate formed, the copper sulphate solution turns green and Copper deposits on Iron nails, giving them brownish colour