Open floor plans are tricky. They can be one color, but often I can find ways to add another color. You can use a second color to define a space, such as the dining or living room. Much depends on the architecture. Is there a natural break for the color? What always makes me cringe is seeing when clients have changed colors from one room to the next by using tape down the wall or putting up a random piece of moulding. If you need to do that, the color should not be changing. Think about all the walls in the rooms. Is there one or two that can be accent walls? If you do decide to do an accent color, it can be a wall that stretches from one room to the other. It doesn't necessarily need to divide up the rooms.