I would paint the back white. Right now the shelves don't look built in.

I would do white. I would also add some crown or trim at the top. I agree that one great piece of art would look better than the three mirrors. It would look fresher and more streamlined. Very nice basement!

white

I don't love the idea of white or tan. I'd rather see a pop of color.

I saw something on Pinterest that I liked: A homeowner cut pieces of styrofoam to fit the "backs" of the openings in her bookshelf ... and covered them with fabric ... then just pushed them to the back of the openings. This means it'd be quick and easy to pull them out and replace them with different fabric, if she wanted to change the color.

I'd also like to see some colorful pillows on that wide hearth.

I love those mirrors! I'd hang them though, which would keep them from "competing" with whatever you've set underneath them.

Yes to crown at the top. It'd add some bulk /importance to the whole thing.

Have you tested the paint in the space yet?

@Anna I have used Manchester Tan in my Kitchen, LR, Stairwell, and in the Foyer which is adjacent to the room in question. I like it because it's neutral and not too "cool" feeling.

I went with white on back of bookshelve!

It's beautiful!