In those days, when that machine was a youngster, people didn't go to a WalMart and buy 10 bobbins at a time, they had probably two at the most and rather than undo the color and rewind another, which was considered wasteful, they would just wind the new color over the other. Some of us grew up as products of the Great Depression and waste not, want not was the rule of the day. To this day, I have a hard time throwing anything away.