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.

These are:
Cable No. 70180 Old Glory Red (equivalent to PMS 193C)
Cable No. 70001 White
Cable No. 70075 Old Glory Blue (equivalent to PMS 281C)