The ions that exhibit color are typically transition metals and some rare earth elements. These ions absorb certain wavelengths of light and re-emit them as visible light, resulting in the observed color. For example, copper ions (Cu^2+) are blue, while iron ions (Fe^3+) can be yellow or orange. The color arises from the electronic transitions within the d-orbitals of the metal ions. The energy difference between the d-orbitals determines the wavelength of light absorbed and, consequently, the color observed.

The data supporting this phenomenon comes from spectroscopic studies, which have shown that the color of transition metal ions is directly related to the energy levels of their d-orbitals. For instance, the absorption spectrum of copper ions peaks at around 500 nm, corresponding to the blue region of the visible spectrum. Similarly, the absorption spectrum of iron ions peaks at around 400 nm, which is in the violet region. These spectral data provide a clear explanation for the color observed in these ions.