Why are the compound of transition elements coloured?

Explanation

(i) Transition metal ions absorb a particular electromagnetic radiation of white light and its complementary colour is transmitted. Example : `Cu^(2+)` ions absorb red colour and transmit blue colour.
(ii) The unpaired electrons present in the d-orbitals of transition metal ions undergo d-d transition by absorbing the energy of certain electromagnetic radiation. Example : `Cu^(2+)`  ion in water absorb red coloured radiation for d-d transitions of electrons. Hence it transmits blue colour.