Curiosity has me googling

In actuality, we were discussing at work why they are called pink slips. Here is what wikipedia says:
"Pink slip refers to the American practice, by a personnel department, of including a discharge notice in an employee's pay envelope to notify the worker of his or her termination of employment or layoff.[1] Receiving a "pink slip" has become a metaphor for the termination of employment in general. According to an article in The New York Times, the editors of the Random House Dictionary have dated the term to at least as early as 1910.[1] The origin of the phrase is undetermined, and there is no evidence that termination notices were ever traditionally printed on pink-colored paper.[2] In the UK and Ireland the equivalent of a pink slip is a P45, in Belgium the equivalent is known as a C4."

So, now we sort of have an answer. It may never have been pink at all!