When adding a new face, joining faces become reflective Having this problem with multiple meshes (so it's probably something easy I'm missing). I have these flat meshes I'm attempting to make into closed 3D objects. But the original vertex shapes become reflective and change colour when adding in a new face.

I'll select the vertexes for the edge of the face (pic 1) create a new face with F key (pic 2). But then BAM, all the faces that connected to those original vertexes have lost their colour and become shiny/reflective (pic 3).
(You can also see the colour change from pic 1 to 2)

I'm wondering if there is a fix for this, or if I'm doing something wrong. Cheers in advance.

Pic 1


Pic 2


Pic 3
The fill option is a bit mischievous some time. You may want to try grid fill and see if the same thing happens. It could be flipping your normals or maybe they are already flipped and it is showing back faced culling. You can check to see this in the material slot, there is a back faced culling viewing option.