For those of us old enough to remember the Beach Boys' song 'Little Deuce Coupe' form the 1960s:
She's got a competition clutch with the four on the floor
And she purrs like a kitten till the lake pipes roar
And if that aint enough to make you flip your lid
There's one more thing, I got the pink slip, Daddy
In other words, "The car belongs to me".
(BTW, a 'four on the floor' meant a 4-speed manual gearbox with a floor-mounted gear lever and 'lake pipes' meant that the car's exhaust pipes were similar to those fitted to racing hydroplanes - or so I was told by a tame American years ago!)
I'm pretty sure that the RN origin of the 'pink chit' is the true one. Some TGIF 'twofers' bar chits were printed on pink paper, as well IIRC?