Why Does The Colour Of Copper Sulphate Solution Change When An Iron Nail Is Dipped In It?

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.