This superstition is based on a historical fact, given that in the past, green paint was considered a lethal weapon.
Throughout the 18th century, synthetic green dyes were entirely made with a brand new compound called cupric hydrogen arsenic. Yes, we’re talking about the same lethal substance you probably know.
Back then, green paint and papered walls were able to release toxic gases when damp, and many factory workers in those times were terribly ill, and some of them even died from the exposure.
Even if the widespread publication of the arsenic recipe in 1822 officially alerted the public about its danger, it seems that the habit has remained up to this day. Some superstitious homeowners see any green wall as nothing but a bad omen.