Before the age of computers, bookkeepers sat at their desks with two bottles of ink in front of them. One bottle contained black ink and the other one contained red ink. A company's profits were always written down in black ink, but all losses and all money owed was recorded in red ink. From that, to be in the red is to be in debt or to be operating at a loss. (来源：英语杂志 http://www.EnglishCN.com)

　　在电脑时代之前，记账人都是坐在桌子边，对着前面的两瓶墨水。一瓶装着黑墨水，另一瓶是红墨水。一家公司的利润通常用黑墨水书写，而所有的损失和欠债都用红墨水记录。从那时候起，to be in the red这个短语就是欠债或者亏损的意思。

sourcec:  信息时报