M.M.
Congrats on the twins! I would paint the walls a neutral brown shade. My sons room has a chocolate brown walls which looks so good with the white crib. The other three walls are a color by benjamin moore called rope.. very nuetral. I personally don't like "themes" for rooms so I would play up the colors. I have a friend who has boy/girl twins and she got these beautiful customized wall decals with each of their names at etsy.com. You can customize.
You could also do a green/white stripe.. that would be very gender neutral. You might need to change the blue/pink color. Pottery barn has good inspiration! Good luck.