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.