Spare yourself the fat by using fat-free mayonnaise and low-fat buttermilk, which has about half the fat as the traditional.
Preheat oven to 425°.
Combine red and sweet potatoes with oil on a baking sheet coated with cooking spray; toss to coat. Bake at 425° for 20 minutes or until potatoes are tender.
Combine buttermilk and next 6 ingredients (through salt) in a large bowl; mix well. Add potatoes and celery; toss well. Serve immediately.