[ Ni (H2O)₆ ] is made up of the Ni⁺² ion, which has a 3d⁸ electrical structure. Because H₂O is a weak ligand, there are two unpaired electrons in this arrangement that cannot pair up. The d – d transition absorbs the incoming light and emits green light as a result. As a result, the solution takes on a green hue.

[ Ni (CN)₄] is made up of Ni⁺² ions that have a 3d⁸ electronic structure. However, the presence of CN^–, a strong ligand, causes the unpaired electrons to pair together. As a result, there is no d–d transition and thus no color.