Deep gray and all the other accents in silvery old wood and cream. This will add drama, allow the other elements to bring ion light and set off the concrete wall as a true feature. If you want light instead of drama, then go for a very pale beige stone. It will echo stone colours in both the concrete and rocks...

I would go with a gold or yellow ... of course I happen to love teal/ turquoise and gold/ yellow together. Something in variations on the photos below... maybe some pink accents if you have a girl or lime green or blue accents if you have a boy.

I have western red cedar shutters and I went dulux antique white. Painted on the wall and ceiling.

Textured linen or with a subtle print wont date. Feature walls make a room smaller as they jump out ar you. Wouldnt recommend. Get some interesting art or craft. Pick up colour with accesories