Lightly grease a 9-by-5-inch aluminum loaf pan with cooking spray. Line the pan with plastic wrap on all sides, leaving a 2-inch overhang; the cooking spray will help the plastic stick directly to the sides of the pan.
In a food processor, pulse the chocolate wafers until very fine, like sand. Add the chocolate sauce and pulse a few more times until the mixture comes together. Pour into the prepared loaf pan and firmly press into the bottom of the pan to create a crust. Freeze for 30 minutes to set.
Meanwhile, remove the chocolate-chocolate chip gelato from the freezer to soften. After 30 minutes, remove the loaf pan from the freezer and spread the chocolate gelato over the crust; use a small offset or rubber spatula to spread the gelato into a flat, even layer. Return the loaf pan to the freezer for another 30 minutes. Meanwhile, let the vanilla bean gelato soften at room temperature.
When the vanilla gelato has softened, transfer it to a bowl and stir in the sliced Amarena cherries, the Amarena syrup and the red food coloring if using. Mix until the cherries are evenly dispersed and the food coloring is completely incorporated. Remove the loaf pan from the freezer and evenly spread the cherry gelato over the chocolate gelato. Place back in the freezer for at least 1 hour more; it takes longer for the cherry layer to set.
Thirty minutes before the cherry layer is set, remove the pistachio gelato from the freezer to soften. When softened, spread in an even layer over the cherry gelato; the pistachio layer should come up nearly to the top of the pan. Cover with plastic wrap and freeze overnight.
When ready to serve, remove the plastic wrap from the top of the pan and invert the spumoni onto a cutting board. Gently lift the loaf pan away and remove the remaining plastic wrap. Slice the spumoni into 3/4-inch slices, using a chef's knife dipped in warm water; dip the knife in the warm water between each slice. Serve immediately.
Amarena cherries can be found in specialty stores or online. If they are unavailable, substitute maraschino cherries in the same amount.