This string is red and not really Jewish.

The Red string is being sold by the beggar women on the approaches to the western wall of the Temple Mount, and they have no basis in Judaism.

The string has been used by the ancient Hindus probably predating the Israelite exodus from Egypt. The Hindus have been using it for almsot the same claimed purpose that charlatans thought up only recently, as an amulet. Maybe they borrowed the concept straight from Indians. 

The youth following the Hassidic charismatic teachings easily get inspiration from the Hassidic amateurs in the form of quotations from Zohar, which tangentially mention a colored string, if translated into English.

A an obscure and little studied book from 400 years ago called coincidentally The Thread Of Compassion (חוט חמלה) speaks of no color, but of attaching a string to one's clothes as a reminder of an elaborately analyzed personal trait, not mentioning any property of the string to guard anyone, nor does it discuss where on one clothes to attach the string.

Some modern-day peddlers claim that the red strings have been snipped from a kilometer-long thread wound around the Rachel's Tomb, thereby absorbing the tomb's magic powers.

None of them mention, together with their cheerleaders, the more learned Zohar fans, that the red strings bear the mystical powers because the midwife attached a red, or crimson, or scarlet string to Judah's newborn son (Genesis 38:28).

But the string was not red, but crimson, and this event deserved the color its own place in Temple observance as commanded by God in Torah.

The crimson string was attached to Zerach, not to Peretz.

Judah was Leah's son, not Rachel's.

There was a great uncertainty as to what red color is. Actually, even black was an uncertain color (Talmud Niddah 20).

Visitors to the Western Wall, however, use common sense. I have never seen any of the throngs to stop and actually buy the string.