From
Wikipedia...
The term pink slip dates to 1915. It is the practice, by a personnel department, of including a discharge notice (printed on pink paper) in an employee's pay envelope notifying the worker of his or her termination or lay-off. Originally the color of the paper had no particular significance.
My knowledge of the term 'pink slip' goes back to the days of drag racing, where the registration (and therefor ownership) of cars were waged. In some states, the registration document was pink, hence the slang.
Cheers! M2