Generally, the worst of teeth stains are due from a person drinking coffee or tea. Which is probably the best bet you could have at this point for giving them a more natural color. I'd say set the teeth in a cup for an extended amount of time. [A few hours maybe?] Then come back to see if any progress has been made into giving them a more bone-colored tone.
Another few suggestions I found were using tobacco tar and rubbing them against the teeth to get the more natural hue that you're looking for. But so far the best route is rubbing off the protective coating of the teeth with an emery board and soaking them in either tea or coffee.