Place the bottom half of the petri dishes on 2 cookie sheets. Place each of the different gelatin flavors into a separate bowl. Bring 3 cups water to a boil. Pour 3/4 cup of the boiling water into each bowl; stir until dissolved. Stir 1/2 cup of cold water into each bowl. Add a few drops of black food coloring to the cherry gelatin to tint it darker.
Divide the gelatins evenly among the petri dishes; 5 per flavor. Chill until set, about 1 hour.
Place 1/4 cup water in a small microwavable bowl. Sprinkle the unflavored gelatin on top; let stand 5 minutes. Microwave until gelatin dissolves, about 25 seconds. Stir in sour cream until smooth. Pour the mixture into a zip-top back; press out excess air and seal. Let cool, kneading bag occasionally, until thickened, about 15 minutes.
Snip a small (1/16-inch) corner from the bag. Pipe amoeba-like shapes on top of your now set petri-dish snacks. Add color with the red or green decorating gels. Refrigerate until set, about 15 minutes. Makes 20 petri-dishes.