Heat the oven to 350 F. Lightly coat a baking dish with cooking spray. Cut the tops off the bell peppers and remove the seeds. Place in the prepared baking dish and set aside.
In a medium skillet, heat the olive oil over medium heat. Add the onion, chopped green pepper and corn. Saute until the vegetables are tender, about 5 minutes. Stir in the chili powder and cilantro or parsley. Reduce heat to low.
In a small bowl, whisk together the egg whites and milk. Add to the corn mixture and stir. Increase heat and continue stirring until egg whites begin to set, about 5 minutes. The mixture should be moist, not dry. Spoon 1/4 of the corn mixture into each pepper. Add the water to the bottom of the baking dish. Cover the peppers loosely with aluminum foil. Bake until the peppers are tender, about 15 minutes. Transfer to individual plates and serve.