The deep blue complex produced by adding excess of Ammonia to ${\mathrm{CuSO}}_4$ solution is

The chemical reaction for the given system can be represented as:

${\mathrm{CuSO}}_4+4{\mathrm{NH}}_3\to \left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_4\right]{\mathrm{SO}}_4$

From the reaction, it is clear that ${\left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_4\right]}^{2+}$ is what causes the deep blue colour to be produced.