I like your idea of "framing" the adjacent rooms, but I'm not really a fan of accent walls, especially with traditional architecture. I'd rather try to achieve that effect by using architectural details than just paint.

Does the rectangular opening have woodwork around it? If not, could you add it? A contrasting trim would be enough to "frame" the adjacent room. I have a trimmed rectangular opening between two rooms in my antique colonial. Originally, there were French doors there, unfortunately long gone. If you have the space, you might also consider adding French doors between your rooms to give them more interest.

Another option, if the opening is wide enough, would be to add columns, or built-in bookcases with columns on top, on the sides of the opening. This would help define the spaces and be in keeping with the style of the house.

If you don't want to mess with carpentry, you could paint one of the rooms a lighter tone of the other. I'm planning to do that with my adjacent rooms. I probably won't replace the French doors because they wouldn't work very well with my furniture placement.

The rectangular opening has woodwork, but it's not very prominent.

DH isn't going to want to pay for carpentry. I really like the idea of different tones. I've already chosen a grayish blue. The hallway is a nondescript oatmeal/toast color. I'm terrible with colors; what would be a good 3rd color?

Thanks everyone!