Test tubes of copper complex ions. The ions of transition metals are coloured. This makes it possible to tell which ion is present in a solution. From left to right: The copper/chloride ion is yellow because most the absorption band is in the ultraviolet part of the spectrum - 390nm. The copper/1, 2 diaminoethane (ethylenediamine) ion is purple because the absorption band is in the green part of the spectrum - 530nm. The copper/ammonia ion is dark blue because most the absorption band is in the red part of the spectrum - 620nm. The hydrated copper ion is pale blue because most of the absorption band is in the infrared part of the spectrum - 800nm.