The Rainbow Series is six-foot tall stack of books on evaluating "Trusted Computer Systems" according to the National Security Agency. The term "Rainbow Series" comes from the fact that each book is a different color. The main book (upon which all other expound) is the Orange Book.
Note (2003): Portions of the Rainbow Series (e.g. the Orange book and the Red Book) have been superseded by the Common Criteria Evaluation and Validation Scheme (CCEVS). For background and further information, see the CCEVS web site here: