According to Air Force Instruction 36-2903:
22.214.171.124. T- Shirt - Desert Sand colored (Tan). The desert sand colored crew-neck t-shirt without pockets is the only t-shirt authorized for wear with the ABU. Exception: Squadron commanders may authorize wear of standardized color morale t-shirts on Friday or during special events (only one color per squadron; individual purchase only, not unit-funded). AF Reserve and ANG personnel, commanders may authorize wear during weekend drills on Friday, Saturday or Sundays or during special events. Wing Commanders are the waiver authority to allow more than one color of T-shirt per squadron if it builds esprit-de-corps and facilitates team building (examples: maintenance and life support personnel, and flight doctors associated with individual flying squadrons). The desert sand colored crew-neck t-shirt without pockets are also authorized for wear with the FDU/DFDU. Synthetic moisture wicking fabrics, 100% cotton or cotton/poly blend may be worn with the ABU as long as they match the color. During flight operations t-shirts must be cotton or fire retardant material due to added protection. T-shirts made of 100% nylon or polyester are not authorized during flight. Installation/squadron commanders may allow for display on morale undershirts organizational emblems, as officially and currently approved, not to exceed 5 inches in diameter, in cloth or silk screen. If displayed, organizational emblems will be worn on the left side of chest of both the desert sand and squadron commander-prescribed Friday morale undershirts. The morale undershirts may also display the organizational emblem on the back of the undershirt in a larger diameter as authorized by commanders. Intheater commander will determine if Airmen are authorized to wear standardized color undershirts with or without organizational emblems. No design or scroll lettering will be visible when wearing the ABU coat (shirt). T-shirt will be tucked into trousers.
See Air Force Dress and Appearance Standards for more information.