A color for my 7yr olds walls?
theme(racing,outdoors,firetruc... you can get stick on wall kits to use and it could encourage his imagination.
a seven-year-old probably has his or her own ideas on what color would be most agreeable for the walls of his or her bedroom. you can always offer another choice if the child's choice is too awful, but let the kid make a decision--it's good practice for life.
Light Blue, with a Wood Trim.
Why not do 2-3 colours. You could "stripe" his room using the green, plus yellow, blue, and red in differing widths. Then put up pics of different things he likes. His bedding would still work in the room but it would have a different energy. Also consider a stripe at about chest-eye height of chalkboard paint. Then he can "change" his room any time he likes by erasing and re- drawing it.