Bring a large pot of salted water to a boil. Cook pasta according to package directions until al dente. Drain and rinse under cold running water to stop cooking. Set aside to cool completely.
While pasta cools, make the dressing: In a medium bowl, whisk together Greek yogurt (or sour cream), mayonnaise, ranch seasoning mix, milk, and lemon juice until smooth. Season with salt and pepper to taste.
In a large mixing bowl, combine cooled pasta, cherry tomatoes, cucumber, red bell pepper, carrots, and red onion.
Pour the dressing over the pasta mixture. Toss gently until everything is evenly coated.
Cover the bowl with plastic wrap or a lid and refrigerate for at least 30 minutes to allow flavors to meld.
Before serving, give the salad a stir. If it seems dry, add a splash of milk to loosen the dressing. Adjust seasoning if needed. Serve cold.