can we get some more info on that? My experience has been quiet the opposite.
I'm fairly certain this is based on a בית יוסף, I heard it from my Rebbe zt"l בשם הגרמ"פ and was quoted above from rabbi belsky from reb moshe. I also heard it from the top OK guy in the west coast, Rabbi klein, very recently, that although some fish are similar, nothing like quite like salmon that is widely available commercially.
this varies wildly
this was heard from a mashgiach in a tuna factory, who surreptitiously took a non tuna fish and put it on the tuna belt post sort. The factory shut down for 4 hours to try to find the culprit and he literally feared for his life if he'd been caught.
For clarity, I am fairly certain that Rav Moshe does not discuss the flesh color.
see above. I'll try to check my notes later