1. Lightly spray an 8.5 x 11-inch baking dish with cooking spray.
2. In a large microwave-safe bowl, whisk together red gelatin and 1/2 cup warm water. Microwave on high for 1 1/2 minutes, stirring until the gelatin fully dissolves.
3. Stir 1 1/2 cups miniature marshmallows into the red gelatin mixture until coated. Microwave for 1 minute until the marshmallows begin to melt, then whisk continuously until completely melted and combined. Allow the mixture to cool to nearly room temperature, whisk again, and pour into the prepared dish. Refrigerate until firm (at least 45 minutes).
4. Repeat steps 2 and 3 with the blue gelatin, remaining 1/2 cup warm water, and 1 1/2 cups marshmallows; refrigerate after pouring on top of the red layer until fully set, several hours or overnight for best results.
5. Once both layers are firm, run a knife around the edges of the dish and invert the gelatin block onto a clean, nonstick surface.
6. Cut the gelatin into strips about 3/4 inch (2 cm) wide and approximately 8.5 inches (22 cm) long. Carefully roll each strip into a pinwheel shape.
7. Insert a 4-inch lollipop stick near the middle edge of each rolled strip without pushing through the top, to secure the pinwheel.
8. Lay pinwheels flat on a tray and refrigerate immediately until serving to maintain firmness.