Why are Hulks shorts purple ?
Basic colour theory. Purple and green are opposites so greater contrast, allowing minimal coverage by the shorts to show expanse of green muscular flesh.
Better question: why doesn't Bruce banners pants rip off when he gets angry and turns into the hulk?
Purple goes pretty good with green........it's an art thing.
The purple ink showed up better against the green ink in the comic books. They were originally gray.
It's his favourite colour :D
To match his vericose veins.