When crepes are fully cooled, place a blue crepe in the center of a serving plate. Spread a scant tablespoon yogurt on top of crepe. Top with another blue crepe. Repeat until all the blue crepes are used, then continue with the green, then yellow, then red crepes, stacking as you go. Save the prettiest red crepe for the top layer and leave plain. Reserve about 2 tablespoons yogurt. Carefully transfer stack on plate to freezer; freeze until set, about 1 hour.