NCSC-TG-024 Vol. 2/4: A Guide to Procurement of Trusted Systems: Language for RFP Specifications and Statements of Work – An Aid to Procurement Initiators, 30 June 1993. (Purple Book)
You can download it from the following link: https://packetstormsecurity.com/files/download/14015/NCSC-TG-024-2.html
Source: https://packetstormsecurity.com/files/14015/NCSC-TG-024-2.html.html