Four ammonia molecules are bound to a zinc ion. A white precipitate of zinc hydroxide dissolves in excess ammonia by turning into this complex ion.
The coordination number is 4 and the geometry is tetrahedral. Unlike copper(II), which is square planar at the same coordination number, zinc(II) has a full set of d electrons, so no direction of approach is more stable than another. The ligands therefore take the tetrahedral arrangement that keeps them furthest apart.
The solution is colorless. With the d orbitals full there is no vacancy for an electron to move into, so no visible light is absorbed. Silver(I) complexes are colorless for the same reason.