The official regulation at the Defense Technology Information Center website is General Services Administration "Federal Specification, Flag, National, United States of America and Flag, Union Jack," DDD-F-416E, dated November 27, 1981.
It specifies the colors by reference to "Standard Color Cards of America" maintained by the Color Association of the United States, Inc.
Cable No. 70180 Old Glory Red (equivalent to PMS 193C)
Cable No. 70001 White
Cable No. 70075 Old Glory Blue (equivalent to PMS 281C)