1. Place blue fruit-flavored gelatin in a bowl. Pour in 1 cup boiling water, and stir to dissolve. In a small bowl, stir 1 envelope unflavored gelatin into cold water. Set aside to dissolve, about 10 minutes. Combine plain gelatin with fruit gelatin, and pour into a 9 x 13-inch pan. Refrigerate 1 hour.
2. Repeat with green, yellow, orange, and red fruit gelatins, and 4 more envelopes of plain gelatin, pouring a new layer on when the previous one is completely set. Refrigerate up to 4 days. Cut into cubes to serve.