"Woah!" and turn and smile "That was something else, huh?" And see who responds and possibly strike up a conversation with those who had something to say.
How would you respond if you awoke and found you were now colorblind?
I'd think of my childhood self who used to believe that people saw in black and white up until a certain point, much like how TVs changed from black and white to colour.
How would you respond if extra-terrestrial life made its presence felt?