Angus Guy, Joe, orcow and Beef11 are all partially right. Green cattle can be any from calves to cattle that have been on feed but are still not finished enough to grade select or better.
Consider green cattle to be cattle that are thin but thrifty and healthy. They can be cattle that are going on pasture or feed yard for backgrounding or cattle that have come off pasture or background yard and have ample room to put more meat and finish on in the feed yard.
Many cattle that grade select are considered green because they still have room to put on more finish to grade choice without becoming over fat.