Preheat oven to 425°F. Cut the purple potatoes into halves or quarters. Toss in the oil and arrange in an even layer on a large baking sheet. Sprinkle with half of the salt. Bake for 45 minutes. Flip the potatoes with a spatula halfway through cooking.
In a blender, combine the yogurt, dill, vinegar, garlic, remaining salt and pepper. Pulse until smooth and well combined. Let sit for the flavors to develop while the potatoes are cooking.
Serve the purple potatoes warm with a helping of Greek yogurt + dill dressing and sprinkle with onions and peppers.
You can either serve individually so each person can control how much dressing they want, or toss everything together in a large bowl and serve family style.