Peel and cube purple and Yukon Gold potatoes, keeping them separate.
Place each type of potato in a large pot and cover with salted water; bring both pots to a boil. Reduce heat to medium-low and simmer until tender, 10 to 15 minutes. Drain. Return to individual pots and mash.
Meanwhile, heat milk in a saucepan over low heat, stirring continually until hot; do not scald. Mix in butter, garlic, salt, and pepper; stir until butter is melted and garlic is fragrant, about 2 minutes. Remove from the heat. Pour 1/2 of the milk-butter mixture into each pot of mashed potatoes and stir to combine.
Scoop both potato mixtures into a large serving bowl. Very gently stir the potatoes together for a swirl effect without totally combining them. Sprinkle with green onion and feta and serve.