For crust, in a mixing bowl, add the melted butter to the crumbs and blend well. Press the mixture into the bottom and halfway up the sides of an ungreased 8x8-inch pan. Refrigerate for 30 minutes before filling.
For lemon filling, combine 3/4 cup sugar, cornstarch and salt in a small saucepan. Stir in water until smooth. Bring to a boil over medium-high heat. Reduce heat; add remaining sugar. Cook and stir until thickened and bubbly, about 2 minutes. Remove from heat; stir in butter, lemon zest and, if desired, food coloring. Gently stir in lemon juice. Cool to room temperature, about 1 hour.
For cream cheese filling, beat cream cheese and confectioners' sugar until smooth. Fold in whipped topping and lemon juice. Spread over crust.
Cover cream cheese filling with lemon filling. Refrigerate overnight.
If desired, pipe, dollop, or layer additional whipped topping over lemon filling. Refrigerate until serving.