When do we change back the proches (ark-cover) etc from white to the rest-of-the-year colour? I would have thought right after Simchas Torah or possibly Isru Chag?
Nitey Gavriel (Rosh Hashana pg. 153) brings the custom to have white paroches etc until after Yom Kippur. In Nitey Gavriel (Sukkos pg. 362) he brings the Maharil, Sharey Efrayim and Minhagei Amsterdam who say to put up white paroches etc. on Hashana Rabba - implying that they had already been changed back from Rosh Hashana. However he writes (without citing a source) that the custom of many Chassidic communities is to leave the white paroches up from Rosh Hashana till after Simchas Torah, and that some even have the custom to do so till after Shabbos Breishis.
In the Lubavitcher Rebbe's shul the white paroches were up from Rosh Hashana till after Simchas Torah (although Luach Kollel Chabad it says to change the paroches and sifrei torah covers to white on the days when Tefillas Tal and Tefilas Geshem are recited, implying they were not left as such from Rosh Hashana). (Otzar Minhagei Chabad pg 363)