Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the orzo and lemon juice, and return to a boil. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through, but is still firm to the bite, about 10 minutes. Drain well in a colander set in the sink. Transfer the orzo to a large saucepan.
Heat the olive oil in a skillet over medium heat, and cook the sweet onion and green onion until the onion is translucent, about 5 minutes; stir in the zucchini and squash, and cook until the squash are tender, about 15 minutes. Stir the cooked vegetables into the orzo; season with parsley, salt, and black pepper to serve.