Preheat oven: Preheat your oven to 350°F (175°C) and grease a standard donut pan with non-stick spray.
Prepare dry ingredients: In a large bowl, whisk together the flour, sugar, and baking powder.
Prepare wet ingredients: In another bowl, whisk together the egg, milk, vegetable oil, and vanilla extract until well combined.
Combine wet and dry: Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Avoid overmixing.
Divide and color batter: Divide the batter evenly into three bowls. Leave one plain (white), add red gel food coloring to the second bowl, and blue gel food coloring to the third bowl. Mix each until the colors are evenly distributed.
Layer the batter: Spoon the colored batters into the donut pan, layering them to create a swirl effect. Use a toothpick to gently swirl the colors for a marbled look.
Bake: Bake for 15–18 minutes, or until a toothpick inserted into the center comes out clean.
Cool: Let the donuts cool in the pan for a few minutes, then transfer to a wire rack to cool completely.
Prepare glaze: In a small bowl, whisk together powdered sugar, milk, and vanilla extract until smooth.
Glaze donuts: Dip the cooled donuts into the glaze, letting excess drip off.
Add sprinkles: While glaze is still wet, sprinkle with red, white, and blue sprinkles.
Serve: Allow glaze to set before serving. Enjoy your festive donuts!