Some tuna that was sliced off for us at the deli counter in our supermarket
recently (one of the big four supermarkets), was a much darker colour than
the end face of the whole fish joint. Although the end face was exposed to
the air, it was a much brighter pink colour than what was cut off for us.

I've heard of chicken being 'refreshed' by rogues by dipping in and out of a
dilute solution of bleach. Might some thing similar be going on here to
freshen up the end of the fish joint?

A neighbour also mentioned that some mince beef they bought from there
supermarket was a much brighter colour on the outside to within about four
millimeters of the surface, and then it became a darker colour inside.

Does it not make you wonder what might be going on here?