Different wavelengths of visible light() are absorbed in transition metal complexes.   The wavelength and colour of light absorbed are determined by the complex.

Colours of light

The basic primary colours for light are:

red         green         blue

If no light is absorbed then the solution remains clear.

If red light is absorbed then a mixture of green and blue are left giving a cyan colour.

If green light is absorbed then a mixture of red and blue are left giving a magenta colour.

If blue light is absorbed then a mixture red and green are left giving a yellow colour.

Why light is absorbed

Photons() of light are absorbed by d to d transitions() when an electron goes from a lower energy d orbital to a higher energy d orbital.   The certain energy required to do this change is relative to the wavelength of light absorbed using the equations and is called the Crystal Field Strength:

Energy = Avogadro's Constant() x Planck's Constant() x frequency of light

and

speed of light() = wavelength x frequency

How ligands affect the colour of the light

Different ligands produced different crystal field strength splittings so the ligands attached to a metal ion will determine the colour of the complex.  The following ligands are listing by increasing crystal field splitting: