Set out a 9-inch pie pan and a food processor. Place the graham crackers in the food processor. Lock the lid into place and pulse into a fine crumb. Then add in the melted butter, sugar, and salt. Pulse to combine.
Dump the graham cracker crumble into the pie pan. Use your hands to press it into an even layer over the bottom of the pan and up the sides. Refrigerate until ready to fill.
Wipe out the food processor bowl. Then add in the cream cheese, sweetened condensed milk, and lime juice. Puree until smooth. (Add food coloring here if desired.)
Remove the food processor blade. Scoop the Cool Whip into the lime mixture. Use a spatula to fold the Cool Whip into the mixture, until very smooth.
Pour/scoop the filling into the pie crust. Then place in the freezer for at least 4 hours, undisturbed.
When ready to serve, sprinkle fresh lime zest over the top, if desired. Cut into pieces while frozen. Let each piece sit out at room temperature for 10 minutes, then serve slightly softened.
Key limes can be hard to find in some regions, but you can usually find bottled key lime juice in the baking or cocktail/beverage section of the market.If you can't find lime-green food coloring, mix 1 drop of standard green food coloring to 2 drops of yellow, until the desired color is reached.