draft papers; call-up notice
Historically, the red paper used for military draft notices in Japan, especially during WWII. Often carries a strong cultural and emotional weight.
See also: 召集令状
During the war, when the red draft notice arrived, you had to go to the army.
red paper
Original literal meaning; simply paper that is red in color. Less common in modern usage except in specific contexts like crafts or traditional items.
For origami, there are various colors like red paper and blue paper.
Compound of 赤 (red) and 紙 (paper). The draft notice sense arose because the official documents were printed on red paper.