Patriotic Potato Salad

Place the red-skinned and white potatoes in one large pot and the purple potatoes in a separate pot (to keep them from tinting the others). Cover both batches with lightly salted cold water. Bring each pot to a boil. Reduce the heat and simmer the potatoes, partially covered, until they are fork tender, about 12 to 15 minutes. Drain the potatoes and let them cool, then cut them into 3/4- to 1-inch chunks. In a large bowl, whisk together the mayonnaise, lemon juice, mustard, and celery seeds. Add all of the potatoes, the green onions, and the salt and pepper, tossing gently to coat. Serves about 8.