Hi, it's me again. :3
In the Japanese manga, Green is the boy and Blue is the girl. In the American version, Blue is the boy and Green is the girl. I don't know what it is in other countries.
I think that's because of the whole video game-related thing. Because in America, the Red and Blue versions were released together, but in Japan, it was the Red and Green version. Then the Blue version was released third. However, in America, the Green version didn't even see the light of day.
So I guess Viz decided to make Red's rival Blue after the American video game counter parts, and Green would be the girl because she played a minor role in the R/G/B arc, much like the Green version in America.
-cough- I'm not sure if that made sense. -.-; I can really lecture at times. x.x
As for Yellow's hat, it depends where you get the volumes from. In The Best of Yellow, they never even reveal that Yellow is female. o__o