The two walls in the picture have the same material, and there isn’t even much difference between them, but for some reason one is lighter and one is darker
It is because of the lighting as in the image that you provided it was clear that there was a point light emitting light near to the right wall. So, the left wall was darker than the right wall. To make it equal try removing the point light or add another point light near to the left wall.
I hope this answered your question