The formation of coloured ions by transition metals is due to

Coloured compounds of transition elements are associated with partially filled (n - 1)d orbitals. The transition undergoes electronic unpaired d – electrons undergo an electronic transition from one – d – orbital to another.

During this d – d transition process the electrons absorb certain energy from the radiation and emit the remainder of energy as coloured light. The colour of ion is complementary of colour absorbed by it. Hence coloured ion is formed due to d – d transition which falls in the visible region for all transition elements.