Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, 15 to 17 minutes; drain.
Heat olive oil in a skillet over medium heat; cook and stir onion, red bell pepper, and orange bell pepper in the hot oil until tender, about 7 minutes. Season with 1/2 teaspoon cumin, 1/4 teaspoon chili powder, 1/4 teaspoon garlic pepper, salt, and black pepper; stir to coat.
Mash potatoes, butter, and half-and-half together with a potato masher in a bowl until smooth; season with 1/2 teaspoon cumin, 1/4 teaspoon garlic pepper, 1/4 teaspoon chili powder, and salt. Fold onion-pepper mixture into potatoes.