Why `Gd^(3+)` is colourless ?

Explanation

Gd - Electronic Configuration : `[Xe]4f^(7)5d^(1)6s^(2)`
`Gd^(3+)` - Electronic Configuration : `[Xe]4f^(7)`
In `Gd^(3+)`, no electrons are there in outer d-orbitals. d - d transition is not possible. So it is colourless.