I'll be flat honest... I dont "go in the water"..... I know what swims around down there.
I would think that they, they being red grouper, snappers, pink porgies, vermillions, rock shrimps/crabs... would take on a brownish color and blend in, sorta like fish camoflauge with the surrounding rocks.
Hard to compare fishing line to something like the fish... but I dont know about the red line. They (people that make the red line and magazines that review or do articles on this stuff) do say at a certain depth (3 ft or more) that the line does blend in (hence the perception that it disappears) with it's surroundings... but usually the deeper you go the darker it gets anyway (the water column that is).
So, I dont know... I'm not convinced. I've never spooled my gear with it, but have used other folks gear that do have it spooled on. Did we catch fish? yes we did... but I've also caught fish on non flouro 400-500lb mono leaders as well.... So for me it's hard to say...
I fish Ande' Backcountry blue line on my trolling gear... I believe in it and it catches fish. Why do I go with blue? I dont know other than I know I do not like pink line or hi-vis line... it's just a personnal preference that really has nothing to to with fishing. Fishing the Back Country stuff though, I have confidence in the line itself and know it works.
Red hooks, do I use them? sure. Oshaunassy (SP?) 7/0 J type hooks in red are my "got to" hooks for baiting squid for mahi on floater lines. They're fished near the top (surface).... why? I dont know other than I know I dont want something "flashy or silver" in my bait while it's up that close to the surface on a drift... so I use red hooks to give the bait more of the look like it's bleeding as most of the time I have the hook exposed... that's just my line of thinking... we catch fish that way,, but we've also used Tru-turn plain ol' silver 7/0's and caught fish that way too...
My take is this, and I'm as particular as anyone else... go with what you believe and have confidence in. If it's working and you feel confident with your gear... then opinions dont matter and you know what works. What's fun though, is seeing what other fisher's use... cause maybe that one guy that just cant seem to hook up,, might switch from a hi-vis line to the red Cajun stuff and have a majical day on the water.
Think he'll spool up the rest of his gear with the red stuff?? go buy red hooks?? or worse, red swivels.... what's next? red line cast nets??
Advertising is a powerfull tool. Tackle monkey is a powerfull draw and know's how to market to feed the addiction. People in the tackle business arent collecting food stamps.......
I think some claims can be exaggerated (but no... fisherman dont lie!!)

but it boils down to what you think works for you.
Bottom line, fish dont care if it's dacron, wire, pink, purple, red, blue, day glo orange... if you have what they want on the end of the hook,, you will catch the fish... just like the fish could care less what type reel reels their scaly butts to the boat.
I know it's a ramble.. but I personnally havent bought off on the idea of the red line, but I do fish with blood red hooks.... Looking at a fishes camo and knowing how bottom fish live,,, I'd say the reds and oranges would turn dark or brownish to blend in with the rocks, reef, corals that they live in and around...
just my thoughts...