What are the colours of the Magical Girl's Rings?
According to the list at the Madoka wiki:


Madoka Kaname (pink) - 
Homura Akemi (purple) - 
Mami Tomoe (yellow) - 
Sayaka Miki (blue) - 
Kyoko Sakura (magenta) - 


You can see in each of the images the name on the back corresponds with the name in the image (which I translated from this online translator). So it seems the colors are the same as the color of their soul gems. It seems the green soul gem also belonged to Madoka Kaname.

The list can also be seen in this Crunchyroll news post.