I threw together a couple Pi Carts with my brothers from the instructions here.
After going through the motions to get RetroPie up and running (with sound via HDMI and no green tint on the GameBoy screen) I figured I'd write up what I had to do in hopes that it will help someone else. I may add to this file as time goes on if I make any more config changes to my RetroPie install. You'll want both a USB controller and a USB keyboard attached to your Pi to make these changes.
NOTE: There are two RetroPie images provided by the official project: RetroPie 0/1 and RetroPie 2/3. This guide is intended for the RetroPie 0/1 image (though you may find it helpful for 2/3).