The crimson thread remained crimson.
There was a Jewish tradition tied to the Day of Atonement, when a scapegoat bearing the sins of the people would be released into the wilderness (see Leviticus 16). Of course, for those who trust in Jesus, this ritual no longer bore any meaning after 30 AD*, as Jesus took our sins upon Himself on the cross. According to the Babylonian Talmud, though, the scapegoat would wear a crimson (red)-colored strap, and it would become white once it reached the wilderness, indicating that God had forgiven their sins:
“How do we know that a crimson-colored strap is tied to the head of the goat that is sent to [the wilderness]? Because it is said, ‘If your sins be as scarlet, they shall be white as snow’ (Isa. 1:18). By a miracle, this crimson-colored strap turned white, thus showing the people the people that they were forgiven of their sins. Rabbi Ishmael says, ‘Now did they not have any other sign? There was a crimson thread tied to the door of the sanctuary. When the goat had reached the wilderness, the thread would turn white…'” (Tractate Shabbat Folio 86a).
“Forty years before the destruction of the Temple, the western light went out, the crimson thread remained crimson, and the lot for the Lord always came up in the left hand. They would close the gates of the Temple by night and get up in the morning and find them wide open” (Jacob Neusner, The Yerushalmi, p.156-157).
“Our rabbis taught: During the last forty years before the destruction of the Temple the lot ‘For the Lord’ did not come up in the right hand; nor did the crimson-colored strap become white; nor did the western most light shine; and the doors of the Hekel [Temple] would open by themselves” (Soncino version, Yoma 39b).
* Actually the act of atonement was transferred to Jesus Christ at that moment in the river Jordan around Yom Kippur 29 AD when the Holy ghost descended upon Him as a Dove. From that moment on He was and is the only One who can forgive sins.