Right. Some people will claim "all my hats are green" is equivalent to "I have no hats which are not green", so he ,must have one hat which is not green. But other people would argue that teh original statement also implies you have at least one hat, so the statement can still be a lie if he has no hats at all. Lewis Carrol explicitly endorses the second view in his book "The Game of Logic". Good luck trying to argue logic with Lewis Carrol.