I always thought the colour of the ring matched the Soul Gem. However after deciphering the runs on the ring I got with a Mami Soul Gem Necklace which said HOMURA, I looked up on the wiki and saw that Mami's is actually purple not yellow like her soul gem.
So I am wondering what are the colors of the rings for each of the magical girls?
EXTRA: I actually brought 6 Ring/Soul Gem Necklace with the 6th set having Homura's Soul Gem and a Green Ring.
EDIT: my mistake, it's a Green Soul Gem and Pink Ring