Add potatoes to a pot with boiling salted water. Simmer for about 15 minutes or until the potatoes are tender. Drain out the water and mash the potatoes.
Preheat the oven to 350 F. Add both cheeses, milk, butter, green onions, onion powder, salt, black pepper, and white pepper. Stir until combined.
Grease a 12-cup muffin tin or two 6-cup tins with non-stick cooking spray. Divide the mashed potato mixture evenly between the 12 cups.
Sprinkle with additional Colby & Monterey Jack cheese. Bake in the preheated oven for about 20 minutes. Remove the mashed potato cups from the oven, sprinkle with green onions, or spoon sour cream on top. Enjoy!