Yes, Josy is sweating. Nevertheless, her skin tone is much lighter than later. Sweating does not make the skin darker, but rather lighter, because the little light would be reflected even more.
In my example, the sweater makes it clear that Josy has two faces. These two images are one and the same scene with two different Josys. One lighter Josy and one darker Josy. Ergo, it was decided that we see either the lighter Josy or the darker Josy.
The same thing happens in the whole sex scene of EP7 that you show us. The MC can fuck Josy whether he is in the throuple relationship or not. Ergo, the decision of EP4 is important because the MC either fucks the Josy from the friendship path or the one from the throuple. Since there is one and the same scene for both paths, DPC took the trouble to give us some clues. That we can have Josy in two different paths and alternate between the light and dark Josy during the sex scene.
To make it more understandable for you, here are two more pictures. Picture 1 shows a normal light Josy and picture 2 shows us an artificially tanned Josy. Now I don't want to start talking about exposure, sweating or anything like that, because Josy's legs are right next to each other and exposed to the same light coming from above, as you can see from the reflections of the legs. So there would be approximately the same shadow as well.
DPC has left itself or us a clue as to what game decisions can do when it comes to relationships.
Light Josy = group of three
Dark Josy = solo or friendship path.
And these two different paths also have different effects, which could result in a Crabs infested Josy because the MC is only friends with her and thus there is then no NTR.