Chlorine has a characteristic smell and it also bleaches things white. Thus, you soon know if you have any chlorine around by its whiff and by testing for its bleaching properties. It has a pale yellow/green colour, although this can be hard to see at low concentrations.
Chlorine bleaches both red and blue litmus paper white. For the animation below the litmus papers were moistened with distilled water and attached to a glass rod before being lowered into chlorine.
It’s possible to see the blue litmus paper turning pink, before it is bleached white. This is because as the chlorine gas dissolves in water it reacts to produce both hypochlorous acid (HOCl) and hydrochloric acid (HCl).
Cl2 + H2O -> HOCl + HCl
Hypochlorous acid is the active disinfectant in water treatment and this is one of the most important technologies used in industrialised societies. Of course, chlorine gas is quite toxic and humans should avoid inhaling it, especially those with known breathing difficulties such as asthma.
What do you predict would happen to a bunch of brightly coloured flowers placed into a gas jar of chlorine?