Question: Am I allowed to wear a red string around my wrist as a segulah for protection against ayein harah, or is it something that was taken from non-Jews?

Answer: It’s permissible, and there’s no concern of imitating non-Jewish customs.
The red string segulah is mentioned by some later authorities, the earliest being Rabbi Vidal Tzarfati in his introduction to Megillat Setarim. See Be’er Moshe (Part 8, Siman 36), who explains that the custom of tying a red string to a baby carriage or crib to ward off ayein harah is deeply rooted in Jewish tradition. Similarly see Avnei Derech (Part 6, Siman 117) who also agrees to this practice.

It’s worth noting, however, that both the Ravivot Ephraim (Part 8, Siman 51) Devaros Eliyahu (Part 9, Siman 59) cite Rabbi Chaim Kanievsky zt”l as opposing this practice. Nevertheless, the prevailing opinion among many authorities allows it, in line with the principle found in the Rashba (Teshuvot HaRashba, Part 1, Siman 9).