.uci extensions are compressed cloop files that are mounted rather than unpacked. These are the safest extensions to use on low ram systems, because they use almost no ram, since they usually exist on a physical drive, and are mounted, rather than unpacked into ram.
Green extensions are named *.tar.gz and are safer to use on low ram systems than yellow and red extensions, because they only write to /home and /opt, which are already writable.
Yellow and red extensions are named .dsl, because they have to write to more areas of the filesystem, and any part of the filesystem that is to be written to must be unpacked to ram. In other words, they can eat a lot more ram.
All .ucis as well as system extensions, like gnu-utils and dsl-dpkg are considered "safe" and rated blue.