Is synthetic astaxanthin (salmon "dye") safe? TL;DR Is synthetic astaxanthin safe for humans to consume?
Background
Synthetic astaxanthin is available as a supplement, and is also commonly consumed by humans in the form of farmed salmon.
Wild salmon eat a lot of shrimp-like krill, which contains lots of astaxanthin, which is what gives salmon it’s red/pink flesh. In farmed salmon, they’re not fed the same diet, so they don’t get the same colour (they’d be grey or off-white). So farms feed them synthetic astaxanthin to give their flesh a red/pink colour.
This says that humans that eat the farmed salmon end up consuming the synthetic astaxanthin via the salmon's flesh.
I found the following:

An article saying:


... one company has announced it will bring a synthetic astaxanthin supplement to market for human use. Their argument for its legality is that it’s already approved as a color additive in food (salmon). This may be a legal loophole that could potentially bring this far inferior supplement onto health food store shelves sometime in the future. The question that remains to be answered is whether or not synthetic astaxanthin is safe for direct human consumption.


Astaxanthin may cause stomach pain in large doses (but so do many foods in large quantities).

Also:



Synthetic astaxanthin is significantly inferior to algal-based astaxanthin

Note that being 'inferior' doesn't imply that it's unsafe.
Question
Is human consumption of synthetic astaxanthin (via capsule, salmon, or any other means) safe?
Yes, the FDA has determined that astaxanthin is &quot;generally recognized as safe&quot; (GRAS), at 0.15 mg/serving. This doesn't indicate whether there are any health considerations associated with it (that's not on topic for this site), but it's not poisonous.