Hard to tell from the photo you posted (just see the corner with the wood wrapping the corner and a peak of stone), but have you considered painting or stining the wood a light grey to pick up the lighter parts of the stone? You could also remove the wood and simply paint the wall without accenting it. The mantel could be painted or stained lighter or darker, beefed up with more or heavier wood... hard to say from that little corner and not seeing the rest of the room or house.

No accent walls, please. They are for people who can't pick art, and/or who don't know how to choose colors which which to accessorize pillows, rugs, objets d'art, etc. The sofa, chair and bench in your photo are fine, but the throw pillow covers are a serious snooze. And please re-think gray as an accent color, it is neither cheerful nor hospitable. If you like ecru. you can have pillow covers in that color with a lot of texture, something along the lines of a cable knit, like an aran sweater. Williams-Sonoma has marvelous throw pillows to give you some inspiration. Look how great those plants look in the photo, green would make a great accent color, and you can add touches of black to ground the room and add sophistication and drama. You can have a black tray on a table, some black and green in art.

Cool house! Oodles of potential! But you've got a lot going on here with the openness and adding a feature wall colour will not be doing the architecture any favours. I would keep it simple and go with one colour throughout. I would go with a white for the walls and the currently painted trimwork. Let the wood accents be the star of your house. That being said, there are 2 things that bother me in the photos. First, the dining room, why does it have a railing? Looks like a weird adult playpen. I would remove that railing and build wall-to-wall steps on each side of the dining room that go down to the lower levels and that would make your space more open and accessible. The other thing that bothers me is the very traditional furniture and lighting. I couldn't tell by your description if the furniture is yours or the previous owners. I would definitely replace all the lighting with something in a more contemporary style. Your furniture should also be simple lines and more minimal. You have an opportunity to make this a really special house. The architecture is already cool, what you do with it could make it awesomely cool!!

I've done accent walls, I think a wall with a fireplace would be a good one for an accent wall but as previously said, I don't think there would be enough contrast between the accent wall and the others with the colors you have chosen. It may not be a "thing" in the U.S. but in the U.K. they embrace the accent wall, at least according to the design shows I have seen. But pictures are necessary to see the fireplace wall.