Zest the oranges using a fine grater.
In a mini food processor, combine the orange zest and granulated sugar. Blitz until the zest is finely chopped and well incorporated into the sugar.
Juice the oranges to get 1/3 cup of juice.
In a small pot, whisk together the egg yolks, whole egg, and orange-sugar mixture until well combined. Add the orange juice and citric acid, continuing to whisk.
Place the pot over medium heat and cook, whisking constantly, until the mixture thickens enough to coat the back of a spoon and retains a clear path when a finger is drawn across it. The ideal temperature is 165°F; use a cooking thermometer to check.
Remove from heat and transfer the mixture to a bowl. Add the chilled butter pieces, whisking until the butter is completely melted and incorporated.
If desired, strain the curd through a metal strainer to remove any zest pieces.
If using, add a few drops of orange food coloring to achieve a vibrant color.
Transfer the curd to a container and refrigerate for at least 1 hour before serving.