Stir together 4 teaspoons cold water and 2 drops of desired food coloring. Combine sugar and colored water in a zip-top plastic bag; blend well.
Pack sugar mixture firmly and evenly into 2 large half-egg molds; scrape off excess sugar mixture. Invert eggs onto a baking sheet; remove molds. Using a spatula, gently shave a flat edge on wide end of each egg to form a base.
Bake at 200° for 15 to 20 minutes or until eggs harden. Remove from oven, cool, and scoop excess sugar from center of each egg half with a spoon or paring knife, leaving a 1/2-inch border. Smooth edges with a spatula or spoon. Return to oven, and bake 10 to 15 minutes or until dried.
Scoop out center of 1 egg half gently, all the way through, with a spoon or paring knife to form an oval window, leaving a 1/2-inch border. Smooth window edges with a spatula or spoon.
Invert both egg halves onto a baking sheet. Pipe Royal Icing into egg half without window, and decorate Easter scene as desired.
Pipe Royal Icing around inside rim of window, and gently place on top of other half, forming a whole egg. Pipe a border around outside rim of window and seam of egg. Let stand at least 1 hour to dry. Store in an airtight container.
For horizontal Egg Kaleidoscopes: Prepare egg molds as directed, and invert onto a baking sheet, removing molds. Gently cut off 1 inch from narrow end of each egg half to form a window. Bake as directed. Scoop out center of each egg, leaving a 1/2-inch border. Decorate shells as desired. Pipe Royal Icing around inside rim of 1 shell, and gently place on top of remaining shell, forming a whole egg. Pipe a border around outside rim of window and seam of egg. Let stand at least 1 hour to dry.