Why are some quarters you find painted red or blue?

They're "colorized" by private firms and sold as collectibles at inflated prices. They're technically considered altered coins and have no numismatic value.

During the 1970's, the Garden State Parkway in New Jersey increased their toll from 15 cents to 25 cents. This caused a public outcry, and spurred a grass-roots protest. Parkway users were encouraged to dye or paint quarters red, and use them as tolls to record their protest vote.

I remember that during this decade red quarters proliferated; I would not be surprised if similar protests with similar red-quarters sprang up around the country. Every time I see a red quarter (fewer and fewer these days) I remember those parkway toll protests.

There must be millions of them still in circulation.

Many bars with a jukebox would have a supply of quarters marked with nail polish that were used to make sure there was music when the first customers came in. When the vending company emptied the cash box, these would be returned to the bartender. ANOTHER STORY:

The colored coins appeared soon after the multi-play jukeboxes appear in bars and honky tonks. At the time one could play 1 song for a nickle or 5 for a quarter. The market coins served where used in varity of ways, one was the house dropping one in the jukebox to "Liven The Joint Up" or if when they matched a customer to see who would put the quarter in the box the house lost The house never lost of course since when the coin box was emptied all the red coins where given back to them. FIVE PLAYS FOR A 25 CENTS! Those were the days my friend.