Putting ones balls into their anus and jacking off. While jacking off and nearing ejaculation, the force pops out the balls (like anal beads) therefore making them brown like a Chocolate cadburry egg with cream filling inside. One could also ejaculate onto the hand and then wipe the semen onto the balls making them appear brown and creamy.
I was watching some porn last night and tried "cadburry egging". It was great!