Following is chicken fillet picture and best before was 2 days ago. Why there discoloured? Is it safe to consume?
This looks like it dried out. Presumably, you opened the package a few days ago, removed a few pieces, and kept the remainder in the fridge. It looks like you had another piece atop the two photographed ones, so the top stayed normal and the exposed sides went dry.
For further evidence, touch the darker parts and see if they feel drier than the light pink ones.