Roast the bell peppers over a gas flame or under a broiler, turning, until charred all over. Transfer the peppers to a paper bag and let steam for 10 minutes. Remove the skin, cores, seeds, and ribs and cut the peppers into 1/2-inch dice.
In a saucepan, bring 2 1/4 cups of water to a boil. Stir in the couscous and 1/2 teaspoon salt. Cover and let stand until tender, about 5 minutes. In a small bowl, combine the olive oil, lemon juice, cumin, and cayenne and season with salt and pepper.
Fluff the couscous with a fork. Add the roasted peppers, tomatoes, cucumber, olives, scallions and garlic. Pour the dressing over the couscous and toss to combine. Let stand at room temperature for at least 1 hour or up to 4 hours. Just before serving, stir in the parsley.