The color Yellow was added to the Sporting Arms and Ammunition Manufactures Institute (SAAMI) standards for 20 ga in the 1960s
SECTION II – PROCEDURES SHOTSHELL MISCELLANEOUS: BODY TUBE COLOR SAAMI VOLUNTARY PERFORMANCE STANDARDS
43 MISCELLANEOUS: BODY TUBE COLOR
The color yellow has been reserved for 20-gauge ammunition. This ammunition shall have a body tube that is primarily yellow. Yellow shall not be used for any other gauge/bore shotshell body. No other recommendations are made as to the color of service body tubes for other gauges/bores.