Why `Zn^(2+)` salts are white `Ni^(2+)` salts or `Cu^(2+)` salts are blue ?

Explanation

In `Zn^(2+)`, the d-orbital is completely filled whereas in `Ni^(2+)`, and `Cu^(2+)`, the d-orbitals are partially filled. Therefore d-d transition, `(t_(2g)to e_(g))` can take place in `Ni^(2+)` and `Cu^(2+)` but not in `Zn^(2+)`. As a result `Zn^(2+)` salts are unable to show any colour and appears as white whereas `Ni^(2+)` and `Cu^(2+)` form coloured compounds.