1. In a small saucepan, bring the milk to a boil over medium-high heat. Remove from heat.
2. Combine the egg yolks, sugar, cornstarch, and instant pudding and fold into boiled milk. Stir well.
3. Return to heat and cook the mixture until it thickens. Cool well.
4. Beat the whipped topping and mix into the cooled milk mixture. Add the green food coloring and mix well (optional). Add the ground pistachio nuts.