The stickers are to assist people who deliver newspapers, as explained in Watchdog Q&A: Dots on mailboxes assist P-C carriers. This source is related to the USA Today network.
Mark Johnson, distribution director for The Post-Crescent, said carriers place the reflective dots to speed the delivery of newspapers in the early morning hours.
"It helps the carrier identify the type of subscription to the correct subscriber under dark conditions," Johnson said. "Not all customers have their address on the mailbox adjoining, so this helps."
The color-coded dots correspond to the different subscriptions and publications that The Post-Crescent delivers, so that's why you might see more than one dot on a mailbox.
See also this statement by the Franklin Trace neighborhood association:
Many residents have recently replaced or will soon be replacing their mailboxes. When installing a new mailbox, be sure to remove the newspaper subscription stickers from the old mailbox and apply them to the new mailbox. You can also call your newspaper and they can tell the carrier your address and have a new sticker applied. If the sticker is not there, the carrier assumes that there is a subscription change and may stop delivery to your address.
The multi-colored dots on mailboxes are placed there by the newspaper to identify which (if any) papers should be delivered there. Red and White dots are for the Indianapolis Star (soon to be Red only as the Star changes to requiring daily paper subscribers to also take the Sunday paper) and a Blue dots are for the non-Indianapolis papers like Wall Street Journal, Barron’s, Financial Times, NY Times, etc.