Wavelength of red light is absorbed by the complex

When (\mathrm{NH}}_{4} $\mathrm{OH}$) is added to the solution containing (\mathrm{Cu}^{2+}) ions, deep blue solution is obtained. (\mathrm{Cu}^{2+}+4 \mathrm{NH}}_{4}\mathrm{OH} \rightarrow\underset{\text{deep blue solution}}{\left[\mathrm{Cu}\left(\mathrm{NH}_3\right)_4\right]^{2+}}+$4$\mathrm{H}}_{2} $\mathrm{O}$) Hydrated cupric compounds absorb radiations corresponding to red light and the transmitted colour is greenish blue (which is complementary to red colour). Thus, cupric compounds give greenish-blue colour.