The tart shells will be medium brown in color. A white or light brown shell indicates that it is underbaked. Allowing your tart shell to get darker in color will bring out all of the caramelization of the butter and sugar together which is what gives your tart shell flavor! Chef Christophe Rull recommends checking on your tart shell’s progress every 10 minutes while baking. Depending on the oven you are using, the back of the oven may distribute more heat than the front. For instance, if you notice that the shells on the back of the oven are darker in color than the tarts on the front, rotate the baking sheet to distribute more heat to the underbaked shells.