The Microsoft Developer Network, or MSDN as it is commonly abbreviated, is a service provided by Microsoft that allows people to have a wide range of software to evaluate.
Disc Color Codes
MSDN discs are color coded so one can tell at a glance what kind of software might be on the disc.
1996 - 2000
Purple discs are Driver Development Kits, or DDKs.
Teal discs are general discs, containing anything from SDKs to beta operating systems.
Green discs are the Office discs, containing Microsoft Office development tools and/or betas.
Red discs are the Server discs, usually labeled as "BackOffice Server Test Platform".
2001 - Present
Green discs are the Operating System discs, which come in Beta and RTM flavors.
Red discs haven't changed since 1996 and are still the Server discs.
Yellow discs contain development tools or Service Packs.
Blue discs are general-purpose applications.