Use Wall Decals to give a theme. Easy to apply They are easily removed. Don't have to worry about pictures being knocked down and injure the children. Many web sites with ideas are available. I put a little boy kissing a little girls sitting on a tree limb with stars and adjacent wall used "Dream" , On other wall used the larger owl with little owl. My little ones loved imitating the kiss good night and an owl says who who who. Mommie Owl and BabY Owl. They're are many many choices on the web sites.

Decals are "hot" right now which is why I wouldn't use them - they'll soon be last year's trend. How many other guests might enjoy this room? Your paint selection sounds great and I'd build a classic guest room around it with a nod to the occasional tiny visitor with a well appointed crib and an upholstered chest/bench at the foot of the guest bed filled with a great assortment of toys that only Grammy has!

There are some beautiful gray and yellow bedrooms here on Houzz, maybe you could find inspiration from one of these? [houzz=]
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Grey and peacock blue is also sophisticated, yet relaxing.

Congrats on your upcoming title as grandparents!
Which ever way you chose to go re:wall colours etc, you can personalise the baby's area with one of our colourful animal lamps which project their colour onto nearby walls.....they come in blue,pink,green and orange so you can create a typical theme for a boy or girl, or even a neutral feel from our range.
www.luminapets.com