Good question! I believe it's due to the heat of your body, depending on that heat, the mood pendant changes the color matching it. (Sorry, I'm trying my best :))
You can in a sense, when they are focused on a particular thing their ears will be pointing forward, when they are relaxed or listening to something behind them the ears will point in that direction....
Yes the heat on your body changes when your mood changes so the the color changes bluegreen- flirty blue- calm purple- excited green- happy amber- crabby or mad