Before adding aqueous ammonia, the precipitate of copper II hydroxide is a light blue colour. After adding aqueous ammonia, the precipitate dissolves and the solution turns a deep blue colour. This is because the copper II hydroxide reacts with the ammonia to form a complex ion called the copper(II)-tetraammine complex, which has a deep blue colour.

The reaction can be represented as follows:

Cu(OH)2(s) + 4NH3(aq) → [Cu(NH3)4](2+)(aq) + 2OH-(aq)

The copper(II)-tetraammine complex is stable in solution, and so the solution remains a deep blue colour.