Sodium vapour lamps glow with yellow colour. This is due

1. sublimation of sodium to emit yellow colour
2. the emission of excess energy absorbed by sodium atoms, in the yellow region of the spectrum
3. the low ionisation energy of sodium
4. its ability to absorb all other colours except yellow

Sodium Vapour Lamps glow with yellow colour due to the emission of excess energy absorbed by sodium atoms, in the yellow region of the spectrum.