Four ammonia molecules are bound to a copper(II) ion. Adding a little ammonia to copper(II) sulfate solution gives a pale blue precipitate of copper(II) hydroxide; adding more dissolves it again into a deep blue solution. That deep blue is the color of this ion.
The coordination number is 4, yet the geometry is square planar rather than tetrahedral. Most four-coordinate complex ions are tetrahedral, and copper(II) is the exception. Its d orbitals are one electron short of full, so the ion is more stable when the bonds along one axis lengthen and the rest flatten into a plane.
The color comes from d electrons absorbing light. The hydrated copper(II) ion is pale blue, while this ammonia complex is a far deeper blue, because changing the ligand changes the wavelength that is absorbed.