In a small saucepan over high heat, combine orange, lime, and lemon juices with 1/2 teaspoon lime zest, 1/2 teaspoon orange zest, and 1/4 teaspoon lemon zest. Reduce until thick and syrupy; you should have about 1 tablespoon. Remove from heat, and let cool.
In a medium bowl, stir together citrus syrup and yogurts. Season with salt. Cover with plastic wrap, and refrigerate until ready to use. Just before serving, sprinkle with remaining zest.