Longer Answer: Mood rings change color because the liquid crystals in them change color in response to temperature. Although the temperature of your finger, and thus the mood ring, do change temperature in response to your emotions, your finger changes temperature for a lot of other reasons, too. Your mood ring will give erroneous results according to the weather and your health.
Since mood rings are sold as novelty items, a toy or jewelry company can put whatever they want on the color chart that comes with the mood ring. Some companies try to match the colors to what your mood might be for a given temperature. Others probably just go with whatever chart looks pretty.