I know that the color of transition metals is related to the splitting of the d-orbitals. Only a certain wavelength is absorbed in the promotion of electron to a higher d-orbital and therefore the remaining colors are interpreted as a certain color. For example in copper(II) sulphate the red wavelength is absorbed and therefore it the complex appears to be cyan. But shouldn't the same amount of energy be released once the electron drops back to its previous state and releases the energy is gained? What wavelength would this drop correspond to?
Not necessarily. The drop may occur gradually so that the colour is "blinded out" by the nonadsorbed colours. The drop may also occur in stages, so that several low energy emissions occur. Some are outside visible range and others are too weak or go unnoticed.