What is 茜 in the phrase 茜纱窗下?
KEY defines 茜纱 as:


  qiànshā
  red yarn


So, yes, 茜 = 红 here.



David Hawkes English translation in The Story Of The Stone Volume 3 reads:


  rosy-misted casement




Yang Hsien-Yi &amp; Gladys Yang's translation in A Dream of Red Mansions Vol. 2 reads:


  madder-gauze window




It seems as if H. Bencraft Joly never translated this portion of the book.