Down here a patty melt doesn't need to be on dark bread, but the bread definitely needs to be toasted.
A common version of a patty melt here, is between two thick slabs of toasted white bread, called "Texas toast."
Obviously cheese is also necessary, hence the "melt" part.
Oh and all buns are bread, but not all bread are buns. Hamburgers are served on buns, specifically, not just any ol' bread. If you want to depart from a standard hamburger bun, then you'll need to make a distinction in the description, and call out "kaiser roll" or whatever, otherwise you're gonna confuse the customer when you serve what you called a hamburger, on something other than a hamburger bun.