In a large heatproof bowl or a 9x13-inch baking dish, whisk together the flavored gelatin and the unflavored gelatin powder until they are fully combined.
Carefully pour the 4 cups of boiling water into the bowl with the gelatin mixture. Whisk continuously for at least 2 minutes, or until the gelatin has completely dissolved and no crystals remain.
Stir the 1 cup of cold water into the hot gelatin mixture until it is fully incorporated. The mixture will become thinner and more liquid.
Lightly spray the 9x13-inch dish with non-stick cooking spray if you are not using the bowl, then pour the combined liquid mixture into the dish.
Refrigerate the gelatin, uncovered, for at least 4 hours, or until it is very firm to the touch and jiggles as a single solid mass.
To serve, dip the bottom of the dish in a larger pan of warm water for about 10 seconds to loosen the gelatin. Cut it into cubes or use cookie cutters to create fun shapes.