I think it would depend how I'm looking at the exchange student. If they're starting (meaning they're trying to learn NGA from the start) a white belt is appropriate, as is the same type of uniform my students wear. If they are there to dabble and learn a bit (like they would at a seminar), then whatever rank they normally wear is appropriate, with whatever they'd normally train in.I think this is like the dance two guys may do when it comes to picking up a check for dinner - but in reverse. The sensei may offer to let the guest keep his black belt rank, while the other declines and wears white. For myself, everyone from a different style (unless maybe I know them to be a bona fide 5th degree or higher black belt) starts as white, as I have done on occasions. An unwillingness to go in as a white belt tells me something of the person's priorities and ego. But that's just me. To each, his own. I'll let skill, not belt color, tell the story.