I might be wrong, and please correct me if so, but as far as I can tell all books from before 1964 are out of copyright if the copyright was not renewed, which doesn't seem to be the case for the Colored Sacred Harp (or, for that matter, the 1936 and 1960 Denson revisions). Before 1964, copyrights needed to be manually renewed 28 years after publication, but for publications after 1964 this process was made automatic. So copyrights now last for 95 years after publication. (So we'll be waiting a while for the 1966 Denson to be out of copyright...)
--Peter Schinske