In case of transition metal complexes, wide range of colours are one of the most important properties which means some of the portions of the visible spectrum is being absorbed from the white light when it passes through a transition metal complex. Hence, the light emerges is no longer white. The observed colour of the complex is complementary to which is absorbed. And this complementary colour is the colour that is generated from the wavelength that is left over after absorption.

Eg: if the green light is absorbed by the complex it appears red.

According to the crystal field theory Higher the crystal field splitting, lower will be the wavelength absorbed by thecomplex. And there is no splitting and the ligand is absent the compound will appear colourless.