In a large saucepan, bring rhubarb, sugar and water to a boil. Reduce heat; cook and stir until thickened and rhubarb is tender. Cool slightly. Transfer to a food processor; cover and process until mixture is smooth; set aside., For crust, place the flour, pecans, butter, confectioners' sugar and salt in a food processor; cover and process until crumbly. Press onto the bottom and up the sides of an ungreased 9-in. tart pan with removable bottom. Bake crust at 350° for 18-20 minutes or until lightly browned., Meanwhile, in a small heavy saucepan over medium heat, whisk the egg yolks, sugar, lemon juice, zest and rhubarb mixture until blended. Add butter; cook until butter is melted, whisking constantly. Stir in food coloring if desired; pour into prepared crust., Bake at 350° 12-15 minutes longer or until center is almost set. Cool completely on a wire rack. Refrigerate for at least 1 hour. Just before serving, dust with confectioners' sugar.