It is an interesting concept. It wouldn't work for me because artistically I would want to make it rainbow colorful. And, I don't associate colors with negative emotions (I love blue, red, green, yellow, brown, purple, etc.) so none have the association with something like "wow, work was stressful today" or "I'm angry about this" or really any other negative emotion I might feel. Colors generally make me happy. Rainbow hued wheels with moon symbols would be fun to color and make me feel happy though.
For me to accurately track moods, I would need to write a few words next to each moon symbol.