Add the cornstarch to a small bowl. Add the water and mix well to make a slurry. Set aside.
Add the brown sugar to a medium-sized saucepan. Then add the pineapple juice, vinegar, ketchup, and soy sauce; mix well. Bring to a boil over medium heat. Stir the cornstarch slurry and pour it in slowly, whisking the whole time. Cook for 1-2 more minutes or until the sauce thickens.
Stir in a few drops of red food coloring if desired.