Can someone explain the source of the blue color of the solution after the reaction between copper and silver ion?
Only the metal copper is red. Any ion of copper is either green or blue. Usually any copper compound is changed to blue or green. For instance CuSO4 is a deep blue color, and copper oxide tends to have a pronounced green hue to it. In your reaction the Copper and silver are in the electro chemical activity series, which means the silver is able to overpower the copper and make the copper take it's negative ion it left behind. Like if you had silver sulphate or silver chloride, the silver would take the plate out on the copper rod and put the copper into solution as an ion which would recombine with the sulphate or chloride ion to give the colored copper compound
What happend is termed a displacement reaction the position the iron reacts with copper and displaces it. Iron sulphate is formed , copper sulphate turns into copper and plated the nail the change in colour of the answer's because of the formation of FeSO4 extremely of CuSO4. CuSO4 + Fe -----> FeSO4 + Cu
