Aqueous copper sulphate solution (blue in colour) gives: (i) a green precipitate with aqueous potassium fluoride

Copper sulphate exists as [Cu(H2O)4]SO4 . It is blue in colour due to presence of the [Cu(H2O)4]2+ ions. When KF is added water is replaced by fluoride ion and green colour is due to [Cu(F)4]2- ions.

When KCl is added water is replaced by chloride ion and bright green colour is due to presence of [CuCl4]2- ions.

In both the cases water , weak field ligand is replaced by fluoride and chloride ions.