Which colour is developed to the solution when alkaline earth metals are dissolved in liquid ammonia ?

Alkaline earth metals dissolved in liquid ammonia, analogous to alkali metals, ionize to form metal cations and free electrons, which then solvate with ammonia molecules as ammoniated electrons. These electrons are responsible for the deep blue coloration, high conductivity, and paramagnetic behavior of the solution. At higher concentrations, the solution transitions to a bronze appearance and diamagnetic character due to electron association.

$M(s) + (x+y)NH_3(l) \rightarrow [M(NH_3)_x]^{2+}(am) + 2[e(NH_3)_y]^-(am)$

Ammoniated electrons absorb energy in the visible region, resulting in the transmission of the complementary deep blue color.

The correct option is $\boxed{B}$.