In later and revised versions of the Great Wheel cosmology, such as the one following the Second Sundering, the Shadowfell was considered a parallel plane, or an 'echo' of the Material Plane, coexisting with it along with the Feywild. Very little was known about the Demiplane of Shadow other than it was a dim and dismal place. If any were brave enough to cross the Demiplane of Shadow, it was possible to find the borders of other planes of reality. Clerics could use the plane shift spell to travel to this plane. Wizards could shadow walk directly to the edge of the Shadowfell and travelers could use this plane as a transitive plane to traverse many Prime Material miles/kilometers very quickly.
For those traveling through the Ethereal Plane, the 'curtain of vaporous color' for the Demiplane of Shadow was the color silver. The largest of these was called the Demiplane of Shadow, and was made up of both positive and negative energy in equal measure.
Demiplanes formed out of the proto-matter that ebbed and flowed about the Ethereal Plane, creating a finite plane with its own Border Ethereal whenever a critical mass was achieved.
