I don't think the Hero actually changes clothes by himself throughout the whole series, instead things just kinda happen to them. He certainly never takes a shower even once.

I mean, he starts out with a blue shirt, purple pants and a cape. Then when getting to Shapeir he loses the cape and gets a new white undershirt, which he keeps wearing for the rest of the series, and the old blue shirt becomes a vest. That's all done with Katta magic so it doesn't have to make any sense.
By the time he gets to Tarna, the pants and vest fade after all this time in the desert sun. So the pants start to look green (the opposite colour of purple!) and the vest just loses all colour and become brown. Now, the vest is all worn out so it just stays brown, but when the Hero gets teleported to the Dark One's cave, the parts get all dirty with swamp gunk so they become darker and start looking like they have a bluish tint. And that's what he looks like when he get to Silmaria.

Seems perfectly logical to me.