Why do sodium-ammonia solutions appear intensely blue?

The intense blue color of sodium-ammonia solutions is attributed to the presence of solvated electrons. When alkali metals such as sodium dissolve in liquid ammonia, the metal dissociates into positive metal ions and electrons that are solvated by the ammonia molecules.1

This process can be represented by the following equilibrium:
$$\text{metal (dispersed)}\rightleftharpoons {\text{M}}^{+}({\text{NH}}_3{)}_x+e^{-}({\text{NH}}_3{)}_y$$

Physical measurements, including electrical-conductivity studies, confirm that these solvated electrons are responsible for both the characteristic blue color and the electrical conductivity of the solution. As the concentration of the dissolved metal increases, the solution deepens in color before eventually transitioning to a copper-colored solution with a metallic luster, a change associated with the association of solvated electrons into electron pairs.1