Line an 8x8 pan with foil and spray lightly with cooking spray.
Melt butter in a large saucepan over low heat. Add 4 cups marshmallows and stir until smooth. Stir in vanilla.
Remove from heat and fold in cereal until coated. Press into the prepared pan with a buttered spatula. Cool 10 minutes.
Melt white chocolate chips briefly in microwave, stir smooth. Divide into three bowls. Color two bowls red and blue, leave one white.
Drizzle red, white, and blue over the cooled bars in streaks. Let set 15–20 minutes, then slice.