This is because iron takes the sulphate atoms to itself and leaves copper alone
They differ in that the electro negativity of Iron and Copper allows the Copper atoms to replace the Iron atoms on the nail. This results in a mixture of less Copper Sulfate and some Iron Sulfate which would have different color properties. The greens and blues come from the Copper so less copper, less greens and blues.
Because of the acid in the solution!!!!!