01 - Set oven to 350°F and coat a 9x13-inch baking pan with butter or nonstick spray.
02 - Mix brownie batter according to package directions, spread in prepared pan and bake for 25-30 minutes until a toothpick inserted in the center comes out with moist crumbs. Cool completely, then cut into 1-inch cubes.
03 - Whisk instant pistachio pudding mixes with cold milk for 2 minutes until thickened. Stir in food coloring if using and refrigerate until needed.
04 - Beat heavy whipping cream with powdered sugar and vanilla extract in a chilled bowl until stiff peaks form using an electric mixer.
05 - In a trifle dish or glass bowl, place half the brownie cubes, followed by half the pistachio pudding, then half the whipped cream. Repeat layers ending with whipped cream on top.
06 - Sprinkle green sprinkles, chocolate shavings or chopped pistachios over the top layer if desired.
07 - Refrigerate assembled trifle for at least 2 hours to allow flavors to meld and achieve optimal texture.