NCSC-TG-024 Vol. 3/4: A Guide to Procurement of Trusted Systems: Computer Security Contract Data Requirements List and Data Item Description Tutorial, 28 February 1994. (Purple Book)
You can download it from the following link: https://packetstormsecurity.com/files/download/14016/NCSC-TG-024-3.html
Source: https://packetstormsecurity.com/files/14016/NCSC-TG-024-3.html.html