They think skin color was the mark Asshole put on Cain for being a jealous farmer murderer.
Edit to add some context- Cain was a farmer and gave a sacrifice of grain. Abel was a shepherd and gave a sacrifice of sheep. Asshole likes the smell of fried mutton, rejects Cains offering (even if it was just as or more work…)
Obviously it’s just a myth; though it’s one used to establish Israelites as being superior to Canaanites and the other semites in the area.
It’s only with the meeting of people who weren’t brown that the mark became assumed as skin color.