Boil the rice in a large pan of lightly salted boiling water for 10 to 12 minutes, until tender but still al dente. Drain the rice well, rinse thoroughly under cold running water, and then drain again. Set aside to cool while you prepare the dressing.
Make the dressing by whisking all of the ingredients together. After transferring the rice to a bowl, add half of the dressing to it to moisten the rice and cool it further.
Add the scallions, peppers, tomatoes, and parsley to the remaining dressing and toss it well to mix with the rice.