What ring do you use to change your pet cats color in runescape?

Ring of Charos (a).

The (a) means Activated.

You gain the Ring at the end of the quest "Creature of Fenkenstrain", but it only has the ability to let you in to someplace werewolf related, i don't remember exactly. It gets the (a) tacked on right after the beginning of the quest "Garden of Tranquillity", after you do a test at the WOM in draynor, he activates it. Also, you can't change the color of the cat, only choose the color for a NEW one. You must be wearing the ring, then talk to Gertrude.