In a small bowl, combine 1 package flavored gelatin and 1 envelope unflavored gelatin. Stir in 1 cup boiling water until dissolved. Pour into a 13x9-in. dish coated with cooking spray; refrigerate until set but not firm, about 20 minutes.
In small bowl, combine the condensed milk and 1 cup boiling water. In another bowl, sprinkle 2 envelopes unflavored gelatin over cold water; let stand for 1 minute. Stir in 3/4 cup boiling water. Add to milk mixture. Spoon 1 cup creamy gelatin mixture over the first flavored gelatin layer. Refrigerate until set but not firm, about 25 minutes.
Repeat from beginning of recipe twice, alternating flavored gelatin with creamy gelatin layers. Chill each layer until set but not firm before spooning next layer on top. Make final flavored gelatin layer; spoon over top. Refrigerate at least 1 hour after completing last layer before cutting into 1-in. squares.