That bluefin looks pretty drab
The bluefin in the pic is actually not that bad looking. The yellowfin (above it in the pic) doesn't look good though. It looks old to me.
Just because a slice of tuna is brown, it does not mean it is not fresh.
Brown tuna usually is not fresh. It's true that brown tuna can still be fresh but in general brown equals older fish. But you really have to know all the shades of a tuna's meat the cycle it goes through. Fat throws a wrench in the grading process.
And other factors determine the color, including the fat content, species and cut. The finest fresh bluefin, which sells for up to $40 a pound at Tokyo's wholesale fish markets, is not a deep red but a pale pink because of the fine web of white fat that permeates the red flesh. Top-quality toro is often a brownish red.
The finest bluefin will usually be a bright orangish/redish and you can see the fat marbling throughout the meat. Bluefin is a different animal from yellowfin though.
When it comes to yellowfin, you want to see that cherry red/translucent color. The paler it gets (without being treated) the lower the grade is. A #1 will start off cherry red/translucent and then start turning paler red until it gets white to brown. This process will usually take 2-3 weeks for it to go from #1 to #3(brown).
And like you said, carbon monoxide makes tuna (not all tuna though) red. You have to start off with a red tuna for it to work. The color of tuna after being treated is not natural and I can spot it right away. I don't agree with the statement in the article about treated tuna never going brown. Treated tuna does change colors after it goes bad. It will turn paler and get streaks of brown/white/hypercolor as it ages. I see it all the time in the supermarkets.