[swarnava]

Ab v clear n ho aapko to plz click....  

http://vlab.amrita.edu/index.php/images/...=208&cnt=1

alcohol or acetone dissolves the lipid outer membrane of Gram negative bacteria, thus leaving the peptidoglycan layer exposed and increases the porosity of the cell wall. The CV-I complex is then washed away from the thin peptidoglycan layer, leaving Gram negative bacteria colorless.
 you only posted this and it suggests that the outer lps layer is dissolved by alcohol tratment so in question the reason is obviously correct. if the outer lps layer was not dissolved the gram negative would nt have been colorless and counter stained with saffranin