What's the wall on the left look like or what's on it ?

If the fireplace is your focal point as it appears to be in your photos, then yes, it would be a good accent wall. And I like the idea of a dusty teal - make sure it is has a grey tone to look subtle.

I wouldn't, based on these pics, as I think it makes a small wall look smaller.

Here is pic of other wall. There will be a comfy chair ottoman in front of window...soon...natural color with a subtle damask design printed in white.

You won't go wrong with anything in the blues (your dusty teal idea, powder blue, peacock blue, grey/blue). I think you should definitely paint it to give it depth and make your lovely fireplace and French door stand out. Contrast is key in good design and paint is the cheapest way to incorporate it into your space. Lovely house!

studio: what if she painted the entire room a very pale teal?

I prefer accent walls to be unpierced by windows or doors. If the FP was the only thing on that wall I could agree to do an accent color, but you're not going to get much bang out of the slivers of wall between the window, door and FP. If you want an accent wall the one behind the credenza would be better, but I don't know where it goes on the second floor area. Post additional pix of it if you want to consider this possibility.

jabbott2012, that would be a house of blues,indeed! Abrowder, can you move the camera back to include some of the upper angles of the room?

(I think a change in the shades might be all that is needed)