what's d-d transition?please explain in terms of color change in detail

The d-orbitals in the transition elements do not have same energy in their complexes. Under the influence of the ligands attached, the d-orbitals splits into two sets of orbitals having slighty different energies. In transition elements which have partly filled  d-orbitals, the transition of electron can take place from one of the lower  d-orbitals to some higher  d-orbitals within the same subshell.

The energy required for this transition falls in visible region. So when white light falls on these complexes they absorb a particular colour from the radiation for the promotion of electron and remaining colours are emitted.The colour of complex is due to this emitted radiation.

• The relationships between the wavelength of light absorbed and the colour observed in some coordination entities are listed in the given table.

Coordination entity	Wavelength of light absorbed (nm)	Colour of light absorbed	Colour of coordination entity
[CoCl(NH3)5]2+	535	Yellow	Violet
[Co(NH3)5(H2O)]3+	500	Blue-green	Red
[Co(NH3)6]3+	475	Blue	Yellow-orange
[Co(CN)6]3−	310	Ultraviolet	Pale yellow
[Cu(H2O)4]2+	600	Red	Blue
[Ti(H2O)6]3+	498	Blue-green	Purple