In most cases a slip is a watered down clay powder applied to the surface of a vessel with a brush or rag, generally to provide an element of color contrasting with the color of the substrate. Sometimes several slips are applied and once all are dry, the surface is carved or scratched and the resulting colors will depend on the depth of the cut or scratch. The same for "marbleized" slips.
There was a period in the late 1800's when a drought in Hopi country forced many families to relocate to the nearby friendly (and wetter) country of the Zunis. The Zuni potters of the time were mostly producing a form of whiteware with a white slip over the surface that was then decorated with various pigmented designs and fired. That white slip would often crack in the firing and today, pots from the place and time are dated by that crackled surface. When the Hopis returned to their mesas near the end of that century, they carried that method of potting with them and it survived until Nampeyo resurrected the Sikyatki method of doing things. Even then, the Naha and Navasie familes still produce whiteware, although they long ago modified their process so the white slip no longer cracks.
Robert Vigil forms his clay body, carves or scratches it with the design he wants, then he slips the form with a watered down micaceous clay before firing it. If he lets the fire burn straight through, he'll have a golden micaceous piece with a couple black fire clouds. If he smothers the fire in manure near the end of it, he'll have a black micaceous piece.
Hector Javier Martinez starts with a white clay body and applies several coats of manganese (black) slip. Once that has dried enough, he begins scratching his designs through that thin black surface using a sewing needle taped to the end of a stick (some Mata Ortiz potters use sharpened bicycle spokes for the same thing).