tripleblessed: My reasoning for making that statement (which was half in jest) comes from the CCSNA website and is as follows:

Ancient Campbell is the same as Black Watch tartan--hardly surprising, since that illustrious regiment is a Campbell regiment, raised by the Duke of Argyll in 1739. (Indeed, as Alastair Campbell of Airds, Chief Executive of Clan Campbell, has indicated in his authoritative book Campbell Tartan, the Black Watch tartan may well have been adopted as the clan's tartan because so many members of the clan were already wearing it, owing to their service in the regiment.)

The Black Watch tartan is today usually made in darker shades of green and blue, while Ancient Campbell is made in lighter, brighter shades. Some Campbells wear the lighter Ancient Campbell for day wear and the darker Black Watch for evening and formal wear--the important point being that they are the same tartan.
Emphasis added. The Black Watch tartan is also known as the Government tartan.

OC Richard: All very true, but not really relevant with regard to the current chiefly-approved Campbell tartans—quite the contrary, in fact—with the possible exception of the Campbell of Loudoun tartan, which seems to have taken inspiration from the Sobieski Stuart's "Campbell" tartan.