In a food processor or a bowl, combine flour and sugar. Add butter, cut into small pieces. Whirl or rub with your fingers until fine crumbs form. Add egg yolk; whirl or mix with a fork until dough holds together. Pat dough into a ball.
Divide dough into 6 equal portions. Firmly press each portion over bottom and sides of a 4 1/2-inch tart pan with removable rim.
Bake in a 300° oven until pale gold, about 25 minutes; let cool.
Meanwhile, peel and segment Melogold, blood oranges, and navel orange.
Spread pastry cream equally in baked crusts. Drain citrus segments; save juice for other uses. Arrange an equal portion of Melogold, blood orange, and navel orange segments decoratively on each filled pastry. Garnish with mint sprigs. Serve or cover and chill up to 1 hour.