After one has made a peanut butter and jelly sandwich, put it in a paper sack, and carried it around all day in a backpack, the bread may have become smashed, thus letting Jelly seap into the holes of the bread. The jelly then changes the bread from white colours to an ugly greyish purple, resembling a bruise on a human.
Hey Zach, you have a jelly bruise today?" "Yeah I do, Jake. I got ran into in the hallway and they smashed my bag