Preheat oven to 375°F (190°C) and line a muffin tin with paper liners
In a bowl, whisk together flour, pistachio pudding mix, baking powder, baking soda, and salt
In another bowl, mix melted butter, sugar, eggs, milk, and vanilla extract until smooth
Gradually add dry ingredients to wet ingredients, stirring until just combined
Fold in chopped pistachios and green food coloring if using
Scoop batter into muffin cups, filling each about ¾ full, then sprinkle extra pistachios on top
Bake for 18-22 minutes or until a toothpick inserted in the center comes out clean
Let muffins cool in the pan for 5 minutes before transferring to a wire rack