The meat of the Site, containment is divided into three sections. Whilst actual SCP objects of course have their own three-tier classification system (rumors of additional tiers are obviously entirely fictitious), the actual containment sector is divided into a completely different system:
Green objects and individuals are considered safe enough that they can be kept in a mere locker, dormitory or holding pen with minimal or zero supervision. Access to these objects is relatively easily obtained, although inappropriate usage will always lead to severe punishment.
Yellow objects are more suspect, and require frequent or constant supervision, often in a specially designed holding pen. The subsector is broken up by periodic doors that can be closed and locked remotely, although their ability to keep out anything but a normal human is debatable. Most (but not all) living objects are yellow at least.
Red objects are bad news. Most are keter, and the rest are the kind of euclid that someone who didn't understand the system would probably want reclassified. The subsector is patrolled by armed guards, the cells are bare and thick-walled, and blast doors thicker than a man is tall are ready to slam shut at a moment's notice. Escape is very, very difficult.