What is the work of Nano2 and HCl in the reaction of liberman nitroso test

While phenol is reacted with NaNO2 and concentrated H2SO4the result will be a deep green or blue colour which changes to red on dilution with water. While generated alkaline along with NaOH original green or blue colour is restored. This reaction is known as Liebermann's nitroso reaction.The reaction is found by Hungarian chemist Leo Lieberman.Libermann nitroso test is performed by scraping off a small amount of the substance and adding a drop of the reagent. The results are analyzed by color of the resulting mixture and by the time taken for the change in color.