Question

when do we get green flame while lighting the gas

Answer 1

Copper, boron  and barium (and probably other) contamination make the flame green. It's most probably copper from the plumbing. Copper salts can be formed when the metal is exposed to corrosive chemicals. I used to play with the spent etching material after creating printed circuit boards, it gave a beautiful green tint to flames.

Other materials give different colors. Sodium in table salt gives orange-yellow, lithium in the Li-ion batteries gives red. (Warning, don't throw batteries into fire! If you want to try, disassemble a completely depleted old battery and remove small amount of the electrolyte for the experiment).