Pour 0.5 cups of cold water into a small bowl, sprinkle gelatin evenly over the surface, and let it bloom for 5 minutes until it appears thick and gel-like.
In a saucepan, heat 1 cup of each fruit juice over medium heat until steaming but not boiling, then remove from heat.
Add the bloomed gelatin to the hot juice, whisking constantly until fully dissolved and the mixture is smooth. If desired, add sugar to taste. Repeat this step for each layer, using different juice flavors.
Pour a thin layer (about 1/4 inch) of the gelatin mixture into the baking dish. Place the dish in the refrigerator and chill for 15-20 minutes until the layer is just set and firm to the touch.
Once the first layer is set, gently pour another layer of a different colored gelatin mixture on top, carefully spreading to avoid disturbing the previous layer. Repeat layering and chilling until the dish is full, approximately 4-5 layers total.
After the final layer has set, run a knife around the edges, then carefully invert the dish onto a serving platter. Slice into squares or rectangles to reveal the colorful stained glass effect. Serve chilled.
