Preheat an outdoor grill for medium-high heat; lightly oil the grate.
Cut two 12x18-inch pieces of aluminum foil and fold them in half to create two 12x9-inch rectangles. Spray one side of each piece with cooking spray. Arrange the potatoes over one sheet of prepared foil in an overlapping pattern, leaving 2 inches free on all sides. Sprinkle with the onion, green bell pepper, red bell pepper, and sliced mushrooms. Season with salt, pepper, vegetable flakes, and paprika to taste. Dot with the reduced fat spread. Place the second piece of foil over the potatoes with the greased side down. Seal the edges by creating several folds; poke 4 to 6 holes in the top to allow steam to escape.
Cook on the preheated grill until the potatoes are tender, 20 to 30 minutes.
