some faces darkened after applying shell modifier

[Anonymous]

i had no problems in my model until i applied shell modifier on the light gray part.

I dont know why it happened, i thought i could choose those polygons and flip them but it won't let me flip.

I attached before and after photos

Can you help please? Thanks a lot

[PropChad]

Is there by chance two vertices overlapping something in that area, or very close to each other? Select all vertices and do a weld at a very low tolerance to check. From the image it looks like a normals direction issue instead of flipped normals, which extra close vertices with a shell modifier can cause issues like that. If that isn't it, if you have a file of just that part, I or someone else can take a peak.