Higher the crystal field splitting energy, lower will be the wavelength absorbed by the complex. The observed colour of complex is the colour generated from the wavelength left over.

In terms of crystal field theory, suppose there is an octahedral complex with empty $e_{\mathrm{g}}$ level, and unpaired electrons in the $\mathrm{t}_{2 \mathrm{~g}}$ level in ground level. If the unpaired electron absorbs light corresponding is blue-green region. It will excite to $\mathrm{e}_{\mathrm{g}}$ level and the complex will appear violet in colour.

In absence of ligand, crystal field splitting does not occur and the substance is colourless. e.g. anhydrous $\mathrm{CuSO}_4$ is white but $\mathrm{CuSO}_4 .5 \mathrm{H}_2 \mathrm{O}$ is blue in colour.