Update:
When I bring the models into the game and use the local game lighting to "light" it up, they appear the same colour but ONLY when I use the lighting. Otherwise, they are still one dark and one light.

Someone is telling me it is like the lighting is inversed similar to how the model was inside out before i reversed normals. It makes sense, is this the issue? And if so, how would I go about resolving it?

I feel like I am overlooking something really silly (this is my first time mirroring/flipping after all)

Here is a photo of them lit-up inside of the game, the previous photos were in the game 'as is'