Could be some new secret camouflage system...
...but more likely is just the low pressure over the wings causing condensation of the airs moisture. You can see the same thing happening at / near the wing tips and leading edge extensions along the body (e.g. F/A 18). When the aircraft is pulling high G, the AoA is pretty high, leading to greater pressure difference between upper and lower wing surface (or lower and upper if inverted
).
... I, eer, think...