In a small bowl, combine pineapple juice, vinegar, ketcup, sugar and salt. Stir until sugar and salt are dissolved. Add cornstarch and stir until well dispersed.

In a sauce pot over medium heat, heat oil. Add bell peppers and cook, stirring regularly until softened. Add liquid mixture and bring to a boil, whisking regularly. Continue to cook, whisking regularly, for about 2 to 3 minutes or until sauce thickens.