The deep blue colour obtained when excess aqueous ammonia is added to a $\mathrm{Cu}{}^{2+}$ salt solution is due to the ion

First a pale blue basic salt or hydroxide forms, which dissolves in excess ammonia as the tetraammine complex $[\mathrm{Cu}(\mathrm{NH}{}_3){}_4]{}^{2+}$, deep blue. The aqua ion is pale blue and the chloro complex is yellow-green.