Rav Belsky was a firm believer
Source?
I am aware that Rav Belsky a"h was opposed to the OU practice of not having a mashgiach temidi for other kinds of fish, but do you have a source saying that he didn't accept the color as a סימן מובהק for salmon?
As far as I was able to ascertain, the only non-kosher fish that might have a similar color would be swordfish, which isn't farmed and is much larger than salmon. There would be absolutely no economic incentive or reason (or supply) to replace salmon with swordfish.
A highly regarded kashrus expert told me that he has a mashgiach temidi at a plant that makes only lox, despite the fact that he believes that even without a mashgiach temidi there is no חשש.