Can you post a picture of the space?

A color would be great. Hard to see from 1 photo which color other than one in your painting.

I love the look of classic white shelves and paint the back wall a deep or bright color. Maybe 4 shades darker than your hallway walls if you want to keep it simple. Personally, I would go for orange. Just remember it is a chore to change that color on a whim.

My first reaction was to go the orange-red in the painting but after staring at it for a while I think I would paint the back of the built-in the color of the blue in the starfish plaque you have sitting on the shelf now. It would keep the space soft and relaxing!

Benjamin Moore's white dove for the built-ins--not sure what the equivalent would be from SW. The inside back of the bookcases might look nice painted in the wall color--also like the idea someone else offered of a deeper gray than your chosen wall color.

The picture is actually the inspiration picture from Houzz... not the actual space... which is currently under construction an is only a wall of 2x4s currently. Here is another Houzz picture showing the hallway color..



None of the painting or built-ins are complete yet. That is being done on Friday. But it is incredibly similar to my inspiration photo in that it is a reading niche off of the main hallway.

I would do white, the same shade as your moldings.

Definitely white for the built-ins. What you will be displaying on the shelves should determine what color and/or texture you do the backs in. If everything on the shelves is very colorful, I would keep it white and maybe do a beadboard back if that would go with the style of the house. If the items on the shelves are of similar colors, I would choose a complementary color for the backs.