Hello Peter,
on my computer screen the two reds are not identical, the top one is clearly darker than the bottom one.
However, that doesn't necessarily mean that one tail light lens is "original" and the other is an aftermarket, or a replacement.
I believe the top one became darker because of long time exposure to the exhaust gases. One can see this quite often when comparing same age left side and right side tail lights, the right side is usually darker.
Also note, it appears that the tail pipes on the lower car stick out more than the other causing less of the exhaust darkening.