Question

Why does a blue color of iron sulphate changes when a peice of iron is dropped in it

Answer 1

I think your question is quite wrong. It should be "a blue colour of copper sulphate"
Anyhow, here is the answer...
blue coloured copper sulphate reacts with iron to make green coloured iron sulphate. So the blue colour of copper sulphate changes when a piece of iron is dropped in it.