I think Belle is most often in her yellow gown at Akershus breakfast. She's not guaranteed, of course, but she's usually the greeter for breakfast, whereas lunch and dinner she'll circulate in her blue dress.
We've never been lucky enough to see her at Hall of Fame. We've seen her there in her blue dress, and all the other princesses, but we've never seen Belle in her yellow gown in Toontown.
We did see her in Fantasyland for Pirate & Princess party night, in her yellow gown. She had a very nice backdrop for photos and I'm happy we were able to see her there!