To the rest of the world, they’re Hawaiian shirts – they’re typically incredibly bright, loud and obnoxious looking, considered to be incredibly casual in every way.
If you come to Hawaii, they become “aloha shirts” which, while they can be vibrant, often are a bit more subdued with a limited color palette, and a focus on patterns that are often (but not always) inspired by traditional Hawaiian clothing and design.
(if you’d like, head over to google image search. Look at “Hawaiian shirt” then look at “aloha shirt”)
As an interesting note, the Aloha shirt is actually considered, in Hawaii, a dress shirt. It’s considered suitable formal attire in many circumstances, and it’s not uncommon to see business professionals wearing them! (I mean, it’s 85 degrees most of the year! who wants to wear a coat and tie all day?)
Give me the shirt, the shorts, the sandals and a floppy hat… I shall make a panda and his name will be Twoflower and I will just wander around in places I don’t belong asking people stupid questions. Then and only then will all of my Discworld dreams be complete.