When you added NaCl in water , you notice that when the water boils over,it causes the flame of the gas burner to turn bright orange.
My question is, how would you explaine the appearance of the color in the flame ?

I think you sort of answered your own question. I would just say the color and perhaps how bright the flame gets. At least, that's how I would do it.

Hi,

Worthbeads, I think the OP is after a physical explanation of the color of the flame.

This is due to the Sodium Ions recombining with electrons in the flame.
These electrons are then given energy by the flame. This raises the electron into a high state.
The electron will then jump down to a lower energy state, as the higher energy state is less stable. When it does this it must lose energy, and it does this by emitting a photon. This photon in the case of Sodium is of a wavelength that corresponds to what we perceive as orange visible light. The energy emitted depends on the difference in energy between the higher state and the lower state. The more of an energy gap that the electron jumps, the higher energy the photon will be, this will appear bluer to us (until it goes into ultra-violet at least).

Hope this answers your question.