The formation of coloured ions by transition metals is due to

Transition element coloured compounds have (n - 1)d orbitals that are only partly filled. 

The electrons absorb some of the radiation's energy during this d-to-d transformation process and radiate the remaining energy as coloured light. Ions have complimentary colours to the colours they absorb.

Therefore, the d-d transition, which occurs in the visible area for all transition elements, results in the formation of a coloured ion.