I think I know the picture your looking for, but have been unable to locate it yet.

In the meantime here is a version of the well known portrait of General John Campbell, 4th Earl of Loudon in the uniform of 64th RoF during the '45 (I have other versions I could scan if needed).

Also, found an interesting page on the STA about The Tartans of Kidnapped, in which they describe the character Colin Roy Campbell of Glenure (The Red Fox) who served in Loudoun's:

The Red Fox Tartan
This is perhaps the most straightforward and the most certain of the Kidnapped tartans. We know that Colin Roy Campbell of Glenure served with Loudoun's Regiment during the '45 Rebellion and in the Low Countries in 1747. Around 1970, researcher Ruairidh Halford MacLeod discovered, in the Huntington Library (U.S.A.), a sample of plaiding which had been "sent to Lord Loudoun for his Highlanders" in 1747, in the Low Countries. This was the standard blue, black, green tartan with a red stripe (with black guards) on the blue and a yellow stripe on the green. Loudoun's tartan was, indeed, the regimental sett which must have been worn by Glenure. In that sense we may think of it as the Red Fox Tartan (it does not seem to have been claimed as a Clan Tartan). This is not to suggest that he was wearing this pattern on the day of his murder, or anything of that sort, only that it was a tartan which we can state with confidence to have been worn by him.

In Stuart Reid's 18th Century Highlanders (Osprey Man-At-Arms series #261) he states:
The red tartan [kilt worn in the Allan Ramsay portrait of Loudoun] appears to be the sett now known as Stuart of Bute, an attribution which may arise from the portrait's being in the possession of that family when the Sobieski Stuart brothers forged their 'Vestiarium Scoticum'....
The waistcoat, largely obscured by gold braiding, is also tartan; this feature also crops up in a portrait of Lieutenant Reid, though the sett may be a simple black on red check of the style known as 'Rob Roy'.

I'll keep searching...