TFWiki wrote:Masterpiece Starscream's creation was overseen by famed Japanese mecha designer Shōji Kawamori, who was particularly intent on hiding the robot-mode details on the underside of the fighter mode. The first prototype for the figure was essentially an upgraded, more complex version of the original toy, with much more accurate robot and jet modes, but Kawamori's desire to avoid simply making a new version of an old toy saw the figure go through a major redesign into its finished form. Perhaps the most notable change was the repositioning of Starscream's tail-fin structures from his legs to his hips, with the intent of creating the feel of a samurai wearing a pair of swords at his waist (presumably, if somewhat oddly, based on the then-recent Armada and Energon cartoons' portrayal of their Starscream as a noble swordsman). Even so, Kawamori was still unhappy with the amount of robotic parts visible underneath the plane, but conceded that too many changes would essentially not make the toy look like Starscream in robot mode. In this pursuit of realism, the most visible concession to the Starscream's design was the abandonment of his traditional color scheme, replaced with an all-over greenish-tinged grey. Given that this color is inaccurate to most real F-15 fighters (whose Gunship Gray coloration is much closer to Starscream's traditional shade), the question of why it was changed at all is not an unreasonable one. However, there are instances of F-15 Strike Eagles that actually utilize this (seldom seen) color scheme.
source articletl; dr The Designer wanted to hide all aspects of SS's robot mode in jet mode, and possibly wanted to homage the "honourable swordsman" role that SS was playing in Armada and Energon.