Another meaning for pink slip was common in California years ago. The title to one's car was pink. To have a pink slip was, to a car owner, a good thing...Are they still pink?

It's actually kind of rainbow-ish; pale green on the left, changing to yellow then to pink on the right. i do seem to recall a smaller, all-pink slip years ago, but i may be thinking of one of the DMV forms that you fill out when you sell the car.

Your Smithsonian curator must not google. i found this with minimal effort:

http://www.nacufs.org/services/publications/journal_1999/woods.asp

"The use of some form of appraisal or evaluation of employee performance is not new. Supervisors at Henry Ford Model T Company conducted daily performance appraisals in a very direct manner. At Ford, when employees finished their workday, they walked past a wall filled with cubbyholes. Each employee had a cubbyhole, and managers put a blank piece of paper in each daily. If a worker was given a white piece of paper it meant that his performance for that day had been acceptable. However, if an employee was given a pink piece of paper it meant that he was not invited back for another day of work; he was fired. This is the origin of the term "pink slip."