Explain why `Zn^(2+)` salts are colourless while `Ni^(2+)` salts are coloured ?

Explanation

`Zn^(2+)` with configuration `[Ar]^(18)3d^(10)` has all filled orbitals. Therefore, its salts are colourless. `Ni^(2+)` with configuration `[Ar]^(18)3d^(8)` has some half-filled orbitals. Therefore, its salts are coloured.