Why does the colour of copper sulphate solution change when an iron nail is dipped in it ?

When an iron nail is placed in a copper sulphate solution, iron displaces copper from copper sulphate solution forming iron sulphate, which is green in colour.

$\underset{Iron}{\mathop{F{{e}_{(s)}}}}\,+\underset{\begin{smallmatrix} 
 Copper\text{ }sulphate \\ 
 \left( Blue\text{ }colour \right)~ 
\end{smallmatrix}}{\mathop{CuS{{O}_{4\left( aq \right)}}}}\,\to \underset{\begin{smallmatrix} 
 Iron\text{ }sulphate \\ 
 \left( Green\text{ }colour \right) 
\end{smallmatrix}}{\mathop{FeS{{O}_{4\left( aq \right)}}}}\,+\underset{Copper}{\mathop{C{{u}_{(s)}}}}\,$

Therefore, the blue colour of copper sulphate solution fades and green colour appears.