That is unusual, locally to me the USAF do have Atlas air, chartered 747s seen used in and out of Mildenhall and Lakenheath in a white commercial scheme but I haven’t seen any grey ones.
A.I says-
The USAF uses grey-painted Boeing 747 aircraft primarily for specialized, low-profile roles, such as the E-4B Nightwatch "Doomsday" command center or in some cases, chartered troop transport. These planes differ from the traditional blue-and-white VC-25A Air Force One, offering a discreet, military-grade exterior.