Flavor 648 is a Watermelon flavor of Mountain Dew. It's similar to Kickstart Hydrating Boost (Energizing Watermelon) and Major Melon, both of which have a similar shade of watermelon flavor. Similar to both of those flavors, Flavor #648's color has a pink-orange look.
This flavor, along with flavors 736, 286, 722, 231, 493, and 509, were shipped to Dew Labs members who were chosen based on their video submissions to the now defunct video website 12seconds.tv, and were shipped boxes of the seven prototype flavors. From the initial seven flavors, taste testers were asked to elect three final flavors for later release at retail stores. 648 eventually lost and was not released to stores.