If you require an explanation, assume this: There are several "standard" uniforms that are typically assigned based on the specific command. These may be chosen by the CO and may have special variants for specific functions (e.g. senior staff). Exceptions may be made for special requests for specific officers, although discouraged (but you're one of the Heroes Of The Universe here, you do what you want so long as you keep breaking Borg faces in). Your own uniform... well, you're the freakin' captain, who's going to say no? Besides, I hear (but do not know for a fact) that generals sometimes can customize their own uniforms quite extensively. Plus, we're looking at a Federation with over 200 planets with numerous societies on each - who's to say their influence didn't creep into the uniform code?