1. Transition metal complex ions have partially-filled d-orbitals at slightly different energies.
2. When electrons move from one d-orbital to another of higher energy (called an excited state), they absorb energy in the visible spectrum equal to the energy gap between their d-orbitals.
3. The colour that transition metal complexes appear is a combination of all the colours that are not absorbed. 

The factors that determine the colour of a complex ion are:

1. The identity of the central metal ion.
2. The oxidation state of the ion.
3. The identity of the ligands.
4. The coordination number of the complex ion.

The observed colour of a transition metal complex is the complementary colour to the colour of light it absorbs. The observed colour is the sum of all the remaining colours that are not observed.

Complementary colours are opposite one another on a colour chart (see above), for example, yellow and blue.

Degenerate orbitals are orbitals that are all at the same energy level.

For example, the five d-orbitals of an isolated transition metal atom or ion are degenerate.