We used red table, green table, and Concord grapes in this tart for a variety of colors.
Heat oven to 425 degrees. Have ready four 4-inch tart rings placed on a parchment-lined baking sheet. Divide dough into 4 equal pieces. On a lightly floured surface, roll each piece of dough into a 6 1/2-inch circle, about 1/8 inch thick. Fit circles into tart rings, leaving about 3/4 inch of dough overhanging.
In a large bowl, combine all grapes, and sprinkle with sugar and flour. Divide grapes among tart rings, and dot each tart with butter. Fold the overhanging pastry over the grapes, leaving the centers exposed. Transfer baking sheet to refrigerator; chill 30 minutes.
In a small bowl, mix egg yolk and heavy cream. Brush exposed pastry with egg mixture, and sprinkle with sugar.
Bake until crusts begin to brown, about 20 minutes. Reduce heat to 350 degrees, and bake until crust is brown and fruit is bubbling, about 35 minutes. Transfer baking sheet to a wire rack to cool, 15 minutes. Remove tart rings; serve warm or at room temperature.