In a medium saucepan set over medium heat, combine the sugar with the cornstarch and add in the juice. Cook, stirring, until thickened and bubbly. Gradually whisk in half of the hot mixture into the egg yolks to temper the eggs. Return the egg mixture to the saucepan and cook, stirring constantly, until the mixture thickens, about 2-3 minutes. Remove from heat and add in the butter pieces. Whisk until melted. Stir in food coloring.