Pink Sheets

What Does Pink Sheets Mean?
A daily publication compiled by the National Quotation Bureau with bid and ask prices of over-the-counter (OTC) stocks, including the market makers who trade them. Unlike companies on a stock exchange, companies quoted on the pink sheets system do not need to meet minimum requirements or file with the SEC. Pink sheets also refers to OTC trading.

Investopedia explains Pink Sheets
The pink sheets got their name because they were actually printed on pink paper. You can tell whether a company trades on the pink sheets because the stock symbol will end in ".PK".