Contrast is a transparency type in the Transparency Tool. Usually you would use it on a separate object to modify the color of underlaying objects (that is, you would use an object as a filter). Depending on the color chosen, it increases the contrast of underlying colors (make them brighter or duller).

The color of the object with Contrast applied:

Gray (below a 50% tint of black) & White - increase contrast (compress the color range) to make the underlying colors brighter.

Gray (above a 50% tint of black) & Black - make underlying colors duller (towards gray).

Other colors - convert the color to the equivalent shade of gray. (Yellow gives light gray, Red dark gray.) Then increase or decrease the contrast of underlying objects.