If you had a gene with a conspicuous effect (like a green beard) that at the same time caused the carrier to favor other individuals with a green beard, you could get a very powerful kind of genetic altruism, one not limited to close relatives. A very strong effect, one that caused you to act as if other carriers were just as valuable as you are (as if other carriers were your identical twin) could exist, but weaker effects (green fuzz) could also be favored by selection – if you were just somewhat more likely to cooperate with others bearing the mark. That could be enough to drive strong selection for the gene, and might not even be terribly noticeable.
This might be especially powerful in humans: we have so very many ways of cooperating or tripping each other up. Now and then you get partial alignment of interests, and remarkable things happen. If we could all just get along, we could conquer the world and make everyone else our slaves and playthings!
A green fuzz system would be more likely to have originated fairly recently (say the Holocene) because population had increased – particularly important with such an unlikely mutation. Probably it’s even more likely to have never happened at all, but similar things have been found in some biological systems.
And you, could course, engineer it in.
Shortly after the Green Beards became influential, you’d see a lot of people wearing fake green beards, which would cut down on the advantage and possibly turn green beards into easy marks, chumps doomed to failure. It would work best if the identifying mark was hard to copy – difficult today, but in the past some things, eye color for example, would have been hard to copy.
This all gets complicated, since it’s not always easy to know what someone else’s best interest is – let along that of the entire Greenbeard race. For that matter it’s not always that easy to know what your own best interest is.
I’m for it, of course: trying to fighting off such a mutant takeover would make life more interesting.