No OP?
One time I got a $20 bill out of an ATM and it had red marks like blood on it, and in red ink, the stamped phrase “If you buy illegal drugs, America’s blood is on your hands.” I figured it was the work of someone who watched that “I helped terrorists by buying drugs” commercial on TV one too many times, and had too much free time and spare cash.
You can guess what I did with that $20 bill the very same night.
Answering the title: I believe they’re put on some bills in large stacks for easy counting. If every hundredth bill has a red mark, you can tell at a glance about how much you have. Probably handy when refilling an ATM machine.
At least, that’s why I heard you used to encouter red quarters in change machines and such…
You encounter “red” quarters in change because proprietors of various establishments which have “juke boxes” or other "house’ machines such as games, from time to time use their own quarters in the machine. They want to know how much they actually take in, so they color their “starter” quarters with fingernail polish and the like so they can say how much of the take was from paying customers.
At least, that’s my take on it.
A couple of cool things I’ve seen on money:
Use the serial number on the bill to see where it’s been! It’s pretty cool.
One of the standard retorts to the PoA debate was “Well, ‘God’ is on our money!” Some replied, “Yep. That’s next”. You can order stamps that read “Trust in yourself” or “Keep church and state separate” with their URL.