Alkali metals impart colour to bunsen flame due to

All alkali metals & their salts imparts colour to flame
Reason: Due to their large size, they have low ionization energies and hence the valency electron easily gets excited when you provide Bunsen frame, when those excited electron returns back to the ground state they emit the excess energy in the form of radiation [Which falls in visible region] and imparts color to the flame
Li - Crimson red
Na - golden yellow
K - Lilac
Rb- Red violet.

cs - Blue violet