Flannel shirts

Flannel shirts are never worn as business or formal dress with a tie. As such, to emphasize their informality, flannel shirts are usually plaid. Another reason might be because they are worn by outdoors people (farmers, loggers, hunters) who apparently always preferred plaid. Because of their working environments, shirts are often quickly soiled and do  not show soiling as easily as solid colors.

I always loved flannel shirts for some reason. They do make them in plain but some things are best as traditional. Rosie gave you a good anser. I also liked your comment about orange. When I lived in Maine, we all had to wear orange if we were going in woods in hunting season as it was a town that drew hunters from all over. Living in the woods...well, you know what that mean.