Hello guys, Peter Griffin here. Today's joke has a little historical context. Let's start with Indozou said in Japanese.
Indozou used 赤紙 (akagami) after looking at the red paper, but why did he say that with such a face? Well, during WW2, the Japanese army sent recruitment notices through mail, and the paper used to write these was red. Let's see an example down here:
These were so prevalent, that after WW2 receiving a red paper became almost synonymous with joining the army.
I couldn't find an example of the paper Comiket sent to Indozou, so I'm taking his word that it was red.