The most commonly used color for meter stamps is red although other colors are often used. With modern digital meters blue is becoming more common and appears to be overtaking red as the most used color. The earliest meters, which could print only a single postage value were required to use ink colors that matched the colors used to print the same denominations of regular adhesive stamps: l¢ green, 2¢ red, 3¢ violet, etc. The only exception was for the 5¢ meter stamp which had to be in red instead of blue. Five cents was the basic first class rate on international mail in the 1920s, and such mail had to be franked in red according to Universal Postal Union regulations at the time.
With the introduction of the first multi-denomination meters in 1929, the Post Office Department relaxed the rules regarding meter ink colors. Multi-denomination meters were allowed to print all stamps in the same color. The only exception was that postage below 2¢ in value could not be printed in red.
In practice soon after the 1920s a wide variety of colors were used indiscriminately with red just being the most common.
Each manufacturer has its own particular shades of colors. By manufacturer they are (from most common to least):
Many other color varieties exist. They are caused by the use of non-standard inks and by mixing inks. In a few instances, multi-colored impressions have been produced by applying different colored inks to separate areas of the ink roller.
In order to speed up the sorting, facing, and postmarking of mail, in the early 1960's the U.S. Postal Service began experimenting with luminescent (fluorescent and phosphorescent) coatings on adhesive stamps. With meters, experiments using fluorescent inks were carried out by Pitney-Bowes in the late 1960's. In September 1972, Pitney-Bowes was awarded a contract from the U.S.P.S to distribute special samples of a machine-detectable postage meter ink to 240,000 meter users. Metered mail using this fluorescent ink can be detected by the same sorting and facing equipment used for postage stamps. Therefore, those pieces of metered mail not bundled by the mailer can be sorted and faced together with ordinary adhesive stamped mail.
Fluorescent ink has been required for all metered mail in the United States since 1 July 1973. Pitney-Bowes' fluorescent ink often shows up poorly as a salmon pink color. Friden's fluorescent ink is frequently a red-violet color.