Wearing a white belt is not a mark of dishonor or punishment. To me, and to most experienced karateka, it represents sho shin --beginner's mind -- which every karateka should maintain, no matter what their rank. Think about it — You're just in town, temporarily, visiting a school that YOU contacted, and you don't know them and they don't know you, and you asked to work out in THEIR dojo, and they're being generous and gracious, in saying yes, you can work out with us. Same style or not, you were not promoted by them -- wear a white belt. If you're a 5th dan, let what you do in class make that clear to all -- why would displaying, to total strangers, your rank, with a piece of cloth, matter so much to you? If you had no belt to wear (you lost it, for example) would you not take the class, even if they said it's fine to not wear one? Would that deter you? if so, ego is the answer. Rank is only relevant/relative within a dojo, usually, and maybe sometimes in its associated/branch dojos. If I never wore a belt for the rest of my days, I would still enjoy training as much as ever. Would you? I hope so! Osu!