As a black walnut fruit ripens, the husk changes from solid green to yellowish green to dark brown. Remember that you are trying to harvest the ripe nuts directly under the tree and ahead of rodents and squirrels.
For most large trees the nuts are too difficult to reach and can only be practically collected after they fall from the tree. Husks must be removed before you store black walnuts and are easier to remove when husking before this green husk turns into a hard black seed cover.
Don't pile walnuts in husks for long periods of time or let the husks deteriorate. Aging walnut husk's juices can penetrate the shell and discolor the nutmeat and give the nut an undesirable flavor. Husk them as soon as is possible after they drop.