Grey: No particular rulership. Sometimes used in place of silver when working with/honoring the Moon. Neutralizes; stops bad luck, gossip, arguments. Slows things down so that you can gain a clearer perspective. Oddly enough, also staves off loneliness and can be used to attract friends & confidants and with meditation.
Great for working with the Fae.
Of course common sense says in general DO NOT LEAVE A CANDLE BURNING UNATTENDED IN AN UNSAFE MANNER. SUGGESTION PLACE CANDLE IN A BOWL OF WATER OR SNUFF OUT WHEN LEAVING YOUR HOME.