Feed sacks were/are generally made out of hessian in this neck of the woods. No chance of making soft undies out of those.There were a few flour sacks made out of plain material with the brand name of the mill stamped all over it and now they are made out of heavy duty plastic or paper.The main memory of our sacks was how the kids used to unravel them to make fake Hawaiian 'grass' skirts for dress-up or parties.
I guess the prints on your feed sacks account for a lot of the colour choices from that era.