USMC: Shirts authorize to be worn under the Utility Uniform?

"The issued utility uniform undershirt is a plain cotton olive green, short-sleeve, crew-neck shirt. Marines may wear optional plain, olive green long- or short-sleeve undershirts of any material, as long as the shirts meet the command’s minimum safety standards and have a Marine Corps approval identification number. Optional shirts (long- or short-sleeve) will meet the minimum requirement, however, commanders may dictate the wear of only short-sleeve shirts when uniformity is required. Wearing the undershirt with the utility uniform is at the individual's option, except that the undershirt will be worn when the utility coat is removed. An undershirt may be required by the commander at such times when uniformity is considered essential such as at formations, ceremonies, or parades." MARADMIN 199/04

No mention of Brown in the regs. Oddly enough I was once issued 3 of those nasty brown shirts for a deployment once.