Hi Helenbar,
I seriously think that you should ask your BS to change a gown for u... something yellowish will not look good on photographs... and the fact that u r tall doesn't mean that you should be limited on yr choice... they should do the amend for u... Unless yr photos are all 3/4 body... otherwise, yr legs would be seen... frankly, it would be nicer to have long and flowy gown in photos rather than short ones... it would definitely not look nice... if they say it's okie, probably means that they would edit the photos after that... but if possible, it's better not, coz not natural anymore.
I agree wif D.O.G. that PS gowns are normally not new but shouldn't be yellowish...
I have a friend who accepted a gown though she finds it old and ugly... for her PS... coz like yr BS, her BS insisted that it would look nice in photography... but in the end, she has to sign up wif another BS to have her photograph retaken...
To play safe, it's better to be firm wif yr BS...