Mix the sugar, cornstarch, and salt in a medium saucepan, off the heat.
Add the grapefruit juice and water and stir with a whisk. A
dd the egg yolks and whisk until the mixture is well blended and of uniform color.
Cook over medium heat for about 5 minutes. The mixture will thicken and start to bubble. It should be thick enough so that it holds its shape when you stir it.
Remove from the heat and stir in the butter and rind. The mixture will be thick and range in color from yellow to pink, depending on the color of the grapefruits.
Cover with plastic wrap to keep a skin from forming on top and refrigerate.