How do I resolve this shading bug with custom faces?

I made some decals for an RP game, and I want to apply them as faces. The script basically just replaces the texture ID in the player's "face" decal. That isn't the problem, though. The problem is that, once the decal is applied, the rest of the Head mesh is darkened.

Here's what I mean.

Raising the transparency on the face decal lightens the skin back to normal, but it also removes the face, so that's kind of counter-productive.

link

Please help me figure out what's causing this.