Screen captures from parade prep footage.
The grey color suggests radio wave transmitting material. The size is not large enough for radar arrays but good for other purposes (e.g., satellite communication).
With the latest photo from by78, we know that there is no place for a booster rocket regardless what that orange surface treatment was for.
Just to be clear which you may have known. xyqq was proposing that the "heat resistant treating" of the wing flap is indication of rocket booster (exhaust), in that line of thinking the boosters have to be under the fuselage not behind. My reply is aiming to that.The tail where the rockets are would have been the obvious spot, to me, not the wings.