The story
From pink + slip (“piece of paper”), referring to the colour of the paper, in some cases as it was in past, and not necessarily present, practice. * (automobile title): Most likely from California, where the title is printed on a smaller index-card sized piece of pink paper; in other states the title is typically a full-size 8 1/2" x 11" sheet, usually printed on blue or blue-green paper with borders, similar to a stock certificate.
Etymology from Wiktionary, CC BY-SA 4.0.