The argument in the comment above is that lab grown salmon cells should not be allowed to be marketed as salmon because they have had components such as beta-carotene added for coloring.

In light of the fact that similar compounds are added to the majority of farmed salmon for similar reasons, the comparison shows that the objection is likely a fallacious appeal to nature, purporting that something is good because it is "natural" or "normal" rather than presenting any objective evidence to argue their point.