Place a rack in the center of the oven and preheat the oven to 350 degrees F. Spray a 8 x 8 square pan with cooking spray and place a sheet of parchment paper inside.
Stir together the graham cracker crumbs, ground pecans, sugar, cinnamon, and melted butter in a bowl with a fork until well combined. Press crumb mixture evenly onto bottom and up sides of the pan. Bake crust for 10 minutes. Remove from the oven. Leave oven on.

In an electric mixer fitted with the whisk attachment, whisk the egg yolks for 2 minutes then add the condensed milk and continue to whisk for another 2 minutes. Add the lime zest to the lime juice along with the food coloring (if using) and with the mixer on low, slowly add to the egg yolk mixture. Whisk until well combined, about 2 minutes. Mixture will thicken slightly when done. Pour filling into crust.

Bake for 15 minutes. Cool completely on rack (the filling will set as it cools). Refrigerate for at least 1 hour, 2 - 3 for best results. Slice into 16 pieces. Serve chilled.