The yellow wrist bands are for a cancer charity, set-up by Lance Armstrong. Live Strong, i think its called.
When I was in the US recently EVERYONE was wearing them - and they were damn hard to get hold of.
They can be bought online:
http://www.nike.com/wearyellow/index_f.html
Or in NikeTown stores.
I'm not sure what the blue ones are for, but I'm sure they're for some other charity as well.
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