A Clone Shadow Trooper (or simply "Shadow Trooper"), is a non-canon type of Clone Trooper or Stormtrooper which was deployed on stealth or special operations (spec ops.) missions.
The canon version of the Shadow Troopers, are the Phase 1 Spec. Ops. Clone Troopers from Stealth Squad, led by Commander Blackout.
During the Clone Wars, they were assigned to the 212th Attack Battalion and were led by Waxer and Boil in the Ghost Company platoon, to whom they used invisibility cloaks to blend-in or become invisible to the enemy in a certain environment. Hence; them literally becoming ghosts or spirits.
There was also another unit which consisted of their own officer, yet the unit was briefly shown in an exclusive comic-book based on exclusive Hasbro action figures.
But during the Galactic Civil War, they were often assigned to stealth missions to destroy Rebel Alliance bases or super weapons. Aside from also being able to be created by using the force; yet this was only featured in the Star Wars: Force Unleashed video game series.
The armor, colors, and markings often vary, and consists of being completely dark black, black with red markings and visors, black with silver markings and visors, and/or dark purple with turquoise markings and black visors.
They are also known as the "Convert Ops." Clone Troopers, as well as the "Utapau Shadow Trooper" from the 212th Attack Battalion.
Star Wars: Shadows Of The Empire comic series (First Appearance)
Star Wars: The Force Unleashed video game series
Hasbro Star Wars action figures