Why does calling someone "yellow" mean cowardly?

Before the United States' Civil War, there were two opposing
sides on the issue of slavery. As the debates between these two
groups continued, agitators on both sides published anonymous
pamphlets and flyers which attacked public figures. These
unsigned publications, which typically contained vicious lies and

rumors, were usually printed on cheap, unbleached paper that had
a yellow tint. When it was learned certain newspaper editors
were contributing to the publishing effort, the practice became
known as "yellow journalism." Eventually, the term "yellow"
became known as a cowardly act.