A theory about the boy with green hair

My grandparents have told me that they knew a boy when they were young.

The boy was born with red hair, but his mother put lye in his hair to give it another colour. The boy was the result of an affair with a married man who refused to recognize the boy as his own. The man also had red hair and the mother didn´t wanted people to see the similarities betweem the boy and the father (but my grandparent said that everybody knew about it, a small society)

I started thinking about it because of in this movie Ingemar´s uncle Gunnar is a redhead. So maybe the greendhaired boy Manne is actually Gunnars illegitimate son.

What do you think?