There are a variety of constructions using different backings and adhesives.
One variation is black gaffer tape, which is designed to be non-reflective and cleanly removed, unlike standard duct tape.
Duct tape is generally silvery gray, but also available in other colors and even printed designs. (source)
Can Guinea Pigs Eat Duct Tape?
Unfortunately, they can’t as they can’t digest it and they may suffer choking on it.
For more foods that guinea pigs can and can’t eat, check out our guinea pig food list.