It could possibly mean literal or figurative meaning, if it is figurative, then it mean describe the book or paper is old and been in moisture and gets molded and yellow.
Many workers receive lots of emails everyday. If your email has an attractive, eye-catching subject line, it is less likely to go unnoticed. This is particularly important if you are engaging in e-mail marketing.
If your message contains sensitive or urgent information, a subject line can help you call this to attention. This means that your email will be opened immediately.
