Transition metal compounds are usually colored. This is due to the electronic transition

colored compound of transition elements is associated with partially filled (n-1)d orbitals. the transition metal ions containing unpaired d-electrons undergoes 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 colored light. the color of ion is complementary of the color absorbed by it.

 hence, colored ion is formed due to d-d transition which falls in visible region for all transition elements.