Strawberry gelatin: Toss 1 pint hulled and quartered strawberries with 2 tablespoons sugar. Let stand 1 hour, then puree in a blender. Sprinkle one 1/4-ounce packet gelatin over 1/4 cup water in a bowl; let stand 1 minute. Heat the strawberry puree in a saucepan over medium heat until hot; whisk into the gelatin mixture until smooth. Cool to room temperature.
Lime gelatin: Combine 2 tablespoons each lime juice and water in a bowl and sprinkle one 1/4-ounce packet gelatin on top; let stand 1 minute. Combine 3/4 cup boiling water and 1/2 cup sweetened condensed milk in a bowl, then whisk in the gelatin mixture until smooth. Cool to room temperature.
Melon gelatin: Puree 4 cups honeydew melon chunks and 2 tablespoons each water and sugar in a blender. Strain through a fine-mesh sieve without pressing the solids. Sprinkle two 1/4-ounce packets gelatin over 1/2 cup of the juice in a bowl; let stand 1 minute. Heat the remaining 1/2 cup juice in a saucepan over medium heat. Whisk into the gelatin mixture until smooth. Cool to room temperature.
How to assemble: Spray a 9-by-5-inch loaf pan with cooking spray, wiping out the excess. Pour in 1/2 cup strawberry gelatin; refrigerate 20 minutes. Repeat with 1/2 cup lime gelatin, 1/2 cup melon gelatin, 1/2 cup lime gelatin and 1/2 cup strawberry gelatin, chilling between layers. Refrigerate the finished mold until fully set, at least 3 hours or overnight.
How to unmold: Bring 2 inches of water to a simmer in a wide saucepan. Run a butter knife or offset spatula around the top edge of the gelatin. Dip the bottom of the pan in the hot water for about 10 seconds, then invert the pan onto a platter. (Redip if it doesn't release after a minute.) Cut into squares.