The skin is exactly the same in both images. It's your perception of the skin that is different based on the surrounding colors.

Our eyes and brain have a remarkable ability to adapt to different lighting conditions. But when the surrounding conditions change our brains expect the things within those conditions to change as well. In this case, the background colors changed drastically, but the skin tones did not change as our brains would expect.

If your model had really been in the middle of the green environment, some of that green would have been reflected onto the model and he would have also reflected it to the camera. As it is, the blue that reflected off the shirt in the first photo is still showing on the skin in the second. So the background has one color of light (green tint reflected by the vegetation) illuminating it and the skin has a different color of light (blue reflected by the t-shirt) illuminating it.

One of the hardest things to do when compositing images is to make sure both parts are being lit by the same type of light so that we don't get mismatches like this.

That's just one reason why proper color management must take into account the ambient lighting conditions under which images are to be viewed. It affects the way our brains perceive color.