Peel the zest off the grapefruits without incorporating any of the white pith and squeeze the juice into a nonreactive saucepan. Cut the zest into strips ⅛ inch wide and 2 inches long. Put the strips in the saucepan with the juice from the grapefruits, the sugar, water, and a small pinch of salt. Bring to a boil over medium-high heat. Reduce the heat and simmer, stirring occasionally, until the zest is tender and the liquid has evaporated to a jamlike texture, about 1 hour. Let cool.