Not sure it's standard but in my town it seems to be private roads (which also have no town road services). There was some issue a few years ago where to comply with E911 regs the town decided they needed to go around renaming common driveways for clarity. So instead of the houses on a shared driveway on Smith Road being 6A, 6B, 6C Smith Road, they renamed the driveway "Jones Way" and numbered them 1, 2, 3. The letter they sent was extremely clear that there is still no municipal plowing or other "road" services and it remains private.

For the identification of private roads, it is going to depend on your municipality, and even then it may not be adhered to consistently. Some municipalities also just have different colored signs by default. Most I've seen are green, but Saugus has a sort of maroon red, for example.

depends on the town. i think all of miltons street signs are red

Sometimes towns use their school colors for street signs so it could just be that the high school's primary color is red.

It depends where you are. Fitchburg and Leominster have red and blue respectively because of the high schools.

This was Fitchburg, actually.

Plympton are all red IIRC

Color is up to the town.

Green and white used to be “standard”, but green blends in with the trees.

Dark red or blue with white lettering is also popular. Some weird places did green signs and yellow letters.

My town switched to dark red (maroon) and white, since our school colors are red and black.

Recently the town looked at the lettering and ran some tests, and we’re switching to upper and lower case letters (rather than all uppercase) since they’re easier to read. (They’re replaced as needed, we didn’t waste money changing all our street signs.)

Guess I hit the nail on the head then