Someone will correct me if I'm wrong, but this might be the so-called "translucent face" Luke, considered an issue of plastic deterioration. Also, note that his limbs are taking on the same color/deterioration quality, but because of the pigment added to them for matching his torso, they aren't showing up quite like the gray of his face. As is pointed out, I believe quite correctly, something like this is likely proven a recent issue of deterioration simply because quality control would have plucked out this guy before circulation. It wouldn't do well to sell a gray-faced Luke back in 1980. Uniformity is the ideal of quality control.