Hello Paolo:
I think you nailed it, just dirty, other causes could be the lighting. The wing upper surface is at an angle to the sun which could be skewing the image, whereas the tail and rudder are in a more direct attitude towards the sun, which appears directly overhead in the photo. The underlying streaking could be bleeding through a bit as well on the cross surround on the upper wing. Your model could be depicting a less weathered airframe than the photo.
Best
Mark