@Argentus and
@T Reiner, I overheard (or overread) your conversation and I'd like to chime in, if I may.
If Argentus is going for the bancho costume in the end, he should imho stay true to the look. That means no jeans or weird color schemes. Banchos are rebellious, gang leading high schoolers. So they are usually wearing Japanese school uniforms, that are sightly altered to their own tastes, showing off their rebellion against the extremely restricting school system and society in Japan, mostly in the 1960s. That's the time frame this special style originated.
The typical black or dark blue "gakuran" school uniforms. Note the pompadours XD :
That cap Argentus gave Mila, was usually also part of a boy's school uniform.
Note the back then typically very long skirts for girls:
Without the school uniform, it's not really a bancho costume anymore. I think, for a Halloween outfit one should be creative of course (and change it up a little), but also stay mostly true to the cliché. That's the one night everybody loves to fulfill a certain cliché.