Preheat oven to 350 degrees F (175 degrees C). Grease an 8x8-inch baking pan.
Make the crust: In a mixer or large bowl, mix the flour, powdered sugar, and salt. Cut in the vegan butter using a pastry cutter or two knives and slowly add in the melted coconut oil until the mixture resembles coarse crumbs.
Press the crust mixture evenly into the bottom of the prepared baking dish. Bake for 25-30 minutes, or until edges lightly golden brown.
While the crust is baking, make the filling: In a medium saucepan, whisk together the granulated sugar, cornstarch, and salt. Stir in the lemon zest, lemon juice, water, and vegan butter. Bring to a boil over medium heat, whisking constantly, until the mixture thickens and becomes smooth.
Remove from heat and stir in the vanilla extract and food coloring, if using.
Wait for both the crust pan and filling to cool completely.
Pour the filling over the baked crust. Cover and refrigerate overnight, or until set.
Sprinkle powdered sugar over pan. Cut into bars and enjoy!