What about primary colors? My DD's bedroom is painted a light green with primary color accessories and is Alphabet themed. There are tons of ABCs and Numbers (1-10) type decorations/accessories you can get for kids rooms.
I'm such a teacher/geek lol.
I have a hatred for theme rooms in general, but I saw a cute B/G twins room on tv. I think I was watching supernanny or something? There were trees painted above each bed. The girl's tree had butterflies and flowers on it, and the boy's tree had a spiderweb and bugs (you could also do owls, birds, etc.) and their names were also in the trees.
You could do really any color walls (beige, gray, green, light blue, yellow) and keep the theme as bugs and flowers... or any "nature" things.