It's customary for a lot of woolen mills to weave the Clark tartan with bright white lines, but the Clergy tartan with more muted grey lines. Like MacWage said, though, this is a modern distinction.

I wrote an article on the Clergy tartan for the Scottish Banner some months ago.

http://albanach.org/clergy_tartan.htm

And here is a kilt I recently made in the modern Clergy tartan (16 oz) by Lochcarron.
http://kilts.albanach.org/clergy1.jpg

Aye,
Matt