Invite Hook #3

Note: this is a mushy puzzle, and skirts the bounds of the 20-question type puzzle. It contains all the information you need to solve it, as long as you make reasonable assumptions about the way the world is set up. If in doubt about an assumption, it is probably wrong or unnecessary. With that caveat:

Marco, the mercurial and merciless president who happily condemned Ricardo to exile (or worse) was on his way home, along with a fine variety of red wine vintages produced on Paradiso, when a gale blew up. Everyone scrambled for the life boats, and Marco wound up on a life boat all his own (none of the sailors wanted him yelling at them). In a piece of backhanded justice, the rest of the crew and passengers were picked up the next day by a passing freighter; not so Marco. He found himself on a little island with a flashlight, some matches, a survival guide, and more wine than he could have drunk in a year. The island initially seemed ideal - it had a number of trees, a fresh-water spring, and a lagoon full of fish.

However, the survival guide informed him that there were two kinds of fish in this area of the sea: ruby red fish, which were poisonous to eat, and emerald green fish, which were excellent eating and had plenty of vitamins. This seemed clear enough, but unfortunately Marco was red/green colorblind. He looked at the fish in the lagoon, but although he could see subtle differences in their neutral shades, he had no idea which were red and which were green.

How can Marco figure out which fish are red, and which green, so that he neither starves nor poisons himself?

Solution