I recently helped my client Kim with some decorating in her sons' room. The two brothers moved in together to make room for their new little sister. We chose Rice Paddy by Sherwin Williams for the walls. It is a nice soft green/yellow color, without being too babyish.
I painted some silhouettes of the boys to hang over their bookcase. You can see my instructions on how to make such silhouettes here.
We arranged some of each boy's artwork, and sentimental collectible pieces over their beds gallery style. We added a monogram to each hanging ensemble that I cut from a scrap of denim, and mounted on a piece of scrapbook paper and framed.
To coordinate the crib bedding with the twin bedding, we added a white bed skirt to the bed that I sewed a brown satin ribbon trim to.
We added some draperies to the windows to help block light for nap time. Using the Lenda curtains from Ikea, I embellished them by sewing a wide blue striped fabric accent onto the drapes to help bring the eye up, and tie in with the blue and white stripe rug that moved from her former nursery into the boy's room.