In a medium saucepan, whisk together the sugar, cornstarch, and salt, then add the reserved cherry juice from the canned cherries.
Bring the mixture to a simmer over medium heat, stirring frequently until thick and bubbly and the cloudy liquid turns into a clear gel, about 5 to 7 minutes.
Stir in the cherries, butter, and food coloring, if using, then cool completely before adding to the pie crust. This can be done up to 1 week in advance.
Preheat oven to 425°F.
On a lightly floured surface, roll out the bottom pie crust and use it to line a 9-inch deep dish pie plate. Spoon the cherry pie filling into the bottom crust.
Roll out the remaining crust and use it to top the cherry pie filling. You can make a lattice crust, or just do a solid crust with slits cut in the top using a sharp knife for venting. Be sure to seal the edges, either by crimping or fluting, then brush the top crust with the beaten egg and sprinkle with sugar, if desired.
Bake for 10 minutes at 425°F, then decrease the oven temperature to 375°F and finish baking for 45-50 minutes, until the crust is golden brown on top and the filling is bubbling. Check the pie around 25 minutes and cover the edges with a pie protector or tinfoil if they are starting to brown too much.
Remove the pie from the oven and cool for at least 3-4 hours before serving.