Initially these bags were plain unbleached cotton with product brands printed on them. In order for women to use these bags they first had to remove the label….http://www.womenfolk.com/quilting_history/feedsacks.htm
In spite of their efforts the entire brand label didn’t always get removed and sometimes it didn’t seem worth the bother especially for making undergarments. As a result there are some amusing stories regarding feedsack underwear. “One young girl was out walking with her beau when she tripped and fell. Oh, how embarrassed she was when her betrothed noticed her underdrawers imprinted with ‘southern best’! Another story was about a woman who made her husband’s drawers from a flour sack and left the words ‘self rising’ on the cloth.”
Here’s one with better pictures of the fabric designs! Some are really beautiful!
More samples of the lovely patterns.
This page has more about the history, and shows a pattern book full of ideas for sewing things using the cotton bags!
Image of a feed sack dress from the Smithsonian: