Whenever people have to put things on their body, they'll do what they can to make it distinctive and fashionable. These folks are all in Hong Kong where they are protecting themselves against the SARS epidemic. Already, in the space of a week or two, fashion has nudged itself into the considerations of which mask you wear.
This makes most sense when you realize that, by wearing the masks, the people here have already given up half of their face - the parts that make people most distinctive, the parts that we relate most to our identities.
Fashion also makes the whole thing a bit less frightening and a bit more interesting. Takes away from the idea that you need to wear the masks, and suggests that you might even want to wear them because they're colorful and interesting and creative. Make a statement -- other than the obvious "I'd like to prevent the spread of disease."
Bonus, slithy_tove sends this along:
...which shows how the practice of mask usage makes intimacy more difficult. Another sad, frightening aspect of the whole thing.