Preheat oven to 350 degrees. Lightly butter 3-quart casserole dish. Boil unpeeled potatoes in salted water until tender but not falling apart. When cool enough to handle, peel and slice. Layer in dish with slivered bell pepper and pimiento. Salt and pepper each layer. Melt butter or margarine in saucepan; add flour and stir until well blended. Gradually add milk, stirring constantly. Add cheeses and melt. Pour over the potatoes and bake 45 minutes to 1 hour.