1. Scrub potatoes and pat dry. Insert a metal skewer into 1 end of each potato.
2. Pour each food coloring into a separate small bowl. Use colorings full-strength for intense color, or dilute with a few drops of water for paler hues.
3. Hold each potato by its skewer and paint food coloring onto the potato skin in designs, as desired, using small watercolor paintbrushes.
4. Lay potatoes on racks to dry, about 10 minutes. Remove skewers. Rub potato skin with oil and set in a 10- by 15-inch pan.
5. Bake in a 375° oven until potatoes are soft when pressed, 40 to 50 minutes. Pile into a basket and serve hot, split open and seasoned with vinegar, butter, or sour cream, and salt and pepper to taste.