Red
On the mood stone ring, red is associated with passion and anger. Red mood stone colors might indicate moments of high adventure, excitement or distress.
Yellow
Yellow is thought to link with the mental process for mood rings. Moments of thought, self-analysis or problem-solving might trigger a mood ring to turn yellow. The color yellow is also associated with the sensation of mixed feelings.
Green
The color green is often associated with the heart's center and can signify growth, healing or change. Some ring-wearers may find that green is the "base" color from which other colors or moods develop.
Blue
Blue moon stone colors often relate to feelings of relaxation or calm. The color blue also relates to the metaphorical throat center, so a blue mood ring might signify that the wearer is struggling to articulate something.
Black
Black mood stone colors might indicate that the wearer is feeling some negative emotions, including tiredness, nervousness or insignificance. Some mood rings turn black when not worn by anyone.
White
White mood ring colors might signify that the wearer feels frustrated, confused or bored. But the color white has some positive associations, too--a white mood ring might signify spiritual growth.
Other colors
Mood rings can sometimes turn other colors that signify different moods. Pink mood stone colors might mean that the metaphoric heart is experiencing growth or change. Purple is linked with sensuality, and orange with envy, motivation or desire. The color orange is also tied to feelings of anticipation.
Mood stones are made from thermotropic liquid crystals, which are sensitive to body temperature. These crystals become calibrated to nearby skin temperatures when worn by someone, and change color as body temperature changes. When someone becomes angry or emotional, body temperatures tend to rise, affecting the color of the mood stone ring. When someone becomes more relaxed, the body temperature drops, again altering the color of the mood stone ring. It's not an exact science, but certainly a harmless (and fun) fashion accessory.