When white light falls on the complex, some part of it is absorbed. 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.
e.g. If green light is absorbed, the complex appears red.
In terms of crystal field theory, suppose there is an octahedral complex with empty level and unpaired electrons in the level in ground level. If the unpaired electron absorbs light corresponding is blue-green region, it will excite to the level and the complex will appear violet in colour.
In the absence of ligand, crystal field splitting does not occur and the substance is colourless.