Love the green. Keep the bottom cabinets white, only because they are the same as the kitchen.

Now for the shelving; because the back of the built-in is actually wall, why not paint it green like the other wall.

For me, the biggest problem is what to do with the shelves. A thought is to stick with one theme...decoys. No need to invest in an Elmer Crowell (my town on Cape Cod). You are in Maine, perfectly acceptable, not beachy or kitschy. One to a shelf.

Check out etsy, thrift shops in your area, etc.

https://www.etsy.com/market/wooden_duck_decoys?order=price_asc

My family had a 19th century home in Gloucester MA. Sitting on the front porch at night, we could see the light from the Provincetown monument and the headlights of cars on RTE 1 Maine coast. That's more than 200 miles apart.

The only issue with going dark on the shelves, is that you then need to find light colored or metallic decor that will stand out in front of the dark color.

I wouldn't go to the trouble of painting, instead I would find some grass cloth wallpaper that I could tack up on the back of the shelves. Relatively easy to do and can be changed out down the road.

Patterned wallpaper behind shelves.

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Grass cloth

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Smart choice on the green. Costal decor has nothing to do with Maine. Are you sure you hate the white cases and it is not just a matter of them being empty so you have tons of white in your face? Do you have lots of books to load them up with?

I think I would find just papering or painting the back of the shelves to be irritating because it will really highlight a white cross hatch pattern from the shelves and boards. Get some paper. Paint it with left over green paint, cut it, and tape it to the back of the cabinets on some of the shelves to see how it looks.

I'd go all or nothing. Paint the whole things green or leave it all white.

Don't paint the white kitchen cabinets.

Do you dislike the white builtins because of the starkness next to the darkness? Because remember, they are empty and once filled won’t be as white.

Can we see more of the space? It’s hard to make suggestions for the white bookcase without seeing the new green walls, the kitchen and the bookcase.

Love the green! Great choice.