In a small saucepan, bring 1/2 cup water and the sugar to a boil and stir until the sugar dissolves, about 1 minute. Remove the syrup from the heat and let cool. Add the cooled syrup to the orange juice; stir in 3 tablespoons of lime juice (or more to taste).
Thread about 6 melon cubes onto each of twelve 10-inch skewers, alternating colors. Pack the skewers in a flat, sealable container and pour the marinade over them. Keep cool until serving.