[Archer]

Can anyone tell me why lithium in anhydrous ammonia goes from blue to almost metallic gold coloured as the lithium to ammonia ratio increases?

I have no idea hoping someone here can explain. It's magnificent to see

[Corribus]

The reason it goes from blue to gold is because as the concentration of lithium (solvated electron) increases, so does the conductivity. There is a threshold value (don't remember what it is), after which the solution essentially becomes metallic, and behaves optically as such.

At low concentrations the color is blue because liquid ammonia has lattice holes that become occupied by electrons. These lattice holes have been modeled as potential energy wells - the electron is essentially quantum confined and behaves like any other quantum confined particles. It has shells and orbitals, as such. A transition between these orbitals creates the blue color. 

All you could ever want to know about it is published here: Zurek et al, A Molecular Perspective on Lithium–Ammonia Solutions. Angew. Chem. Int. Ed. 2009, 48, 8198 – 8232.

[Archer]

Thanks for the reference, I have read it and think I understood :/