Answer:
It varies based on the producer of the ring. Mood rings generally come with slips of paper that list the colors and their corresponding moods, but there are many versions of these slips of paper so the colors for different rings don't always match up.
One very common slip lists:
Black- stressed
Red- nervous
Orange- unsettled
Green- active
Blue- relaxed
Indigo- lovable
Purple- romantic
Note that the color of mood rings is based on body heat, not actual mood, so the moods matching the colors of a mood ring should be taken lightly.