Question

what is the cause of colour of alkali metals.....?

Answer 1

The ionisation energy of alkali metals is very low. The energy of the flame is sufficient to excite the electrons to higher energy level.

The excited state is unstable. Hence, the electrons emit energy and come back to the original levels. This emitted energy falls in the visible region of electromagnetic spectrum. This is why, alkali metals imparts colour to flame.

Answer 2

You see this light as the colour of a specific wavelength . Atoms from different elements have different number of electrons in their electron shell , so photons emitted as these electrons jump back each have different energy levels and therefore emit light of a different wavelength or colour.
Hope it Helps