The Short Answer: No one knows, but the search has been interesting.
The Long Answer: Getting a pink slip usually means you're fired. It's not something most people look forward to. Peter Liebhold, then, is an odd guy. He's been searching for a pink slip for years, and he's disappointed he keeps coming up empty.
Liebhold isn't looking to get canned. Rather, finding a pink slip is his job. He's a curator at the Smithsonian Institution's National Museum of American History and Chair of the Division of Work and Industry.
The history of business culture is his business. The pink slip is a mystery he's been chasing for a long time. Almost everyone is familiar with the phrase, but no one seems to know where it originated, or if there's an actual pink slip out there to be had.
The usual line of reasoning is that the phrase was born when one or more companies started the practice of terminating employees by giving them notice on a piece of pink paper. The color was chosen so that the notice would stand out from the rest of the paperwork on the poor guy's desk and he wouldn't miss it. The catch, of course, is that Liebhold and other historians haven't been able to track down an actual slip, or find any companies that actually fired people like this. The most they had to go on for a while was the Oxford English Dictionary citing the phrase's first known appearance in a 1915 pulp novel about baseball.
The most promising lead Liebhold ever had, he told the Baltimore Sun, was the Ford Motor Company. While poring over an obscure history journal, he found a footnote that led him to another article in another journal that talked about the daily evaluations of Ford's assembly line workers. The workers, the article went, all had lockers or cubbies where they kept their things, and at the end of the day they would find a slip of paper from management there. A white paper meant the day's effort was acceptable. A pink slip, though, meant that they weren't wanted back in the morning.
Liebhold thought he'd finally found his elusive slip, but when he tracked down the source of the story, a California-based management consultant, he learned it was just an anecdote overheard in college. The consultant had been repeating it ever since. Neither the consultant, nor anyone at Ford who Liebhold talked to, had any evidence that the story was true.
Swing and a miss.
Liebhold's search hasn't been in vain, though. He's found a few other bits of workplace history during the hunt, like the first American filing cabinet and some red twill that secretaries used to use to bundle documents together — apparently, the inspiration for bureaucratic "red tape."