Acc to the the crystal feld theory,in the presence of a ligand atom the d subshell splits into two subshells called eg and t₂g (g stands fr gerade). When an electron jumps frm eg to t₂g  or vice versa it is called d-d transition. Due to this transition, the complexes r able to absorb and emit light light of a certain energies (depending on the CSE), thats why they r coloured compounds 

Transition elements are coloured due to the presence of unpaired electrons in the d orbit.

These unpaired electrons undergo transition within the d orbitals. ie.

When the electron moves from one orbital to another energy is released as em waves in visible spectrum. This movement of electrons occurs as the d orbitals has similar energy.