Ok gang, I'm coming to this thread late, but here's my advise.

I think a cummerbund is fine to wear with a PC, but a vest looks better. Either would look better than a belt, and remember than any time you are wearing a vest or cummerbund, you don't wear the belt!

I think it's also ok to experiment with colors other than black, but black is definitely safest. Another color can look smashing, and really make you stand out, so long as it coordinates well with the rest of the outfit -- kilt, jacket, hose, even your skin tone! Have an eye for the overall look of the ensemble. If you don't have a good eye for color coordination, best to stick with black, because you will run the risk of looking really goofy really fast.

About tartan vests, most all the tartan vests I've seen have been the 5 button style you wear with an Argylle, not a PC vest (3 button). Traditionally vests were always made with the tartan on the bias. The vest I'm wearing in the picture Panache linked to was modelled after a vest c. 1800 that we have at the Scottish Tartans Museum. That vest was made in the same tartan to match the kilt, and the front is on the bias. The back is also full tartan, but the tartan there is straight. I have received nothing but compliments on this vest, including when I've worn it with a black formal Argyle jacket. I've also worn it with a brown tweed kilt jacket for day wear and it looks fine.

Most tartan vests available today have the tartan straight, which to my eye does not look as nice. I'd much rather see tartan on the bias.

Now, I have once seen a tartan vest worn with a "PC-style" jacket (not a PC, but close to it) with success. Ruardi Halford MacLeod, first cousin to the cheif of the MacLeods and president of the Clan MacLeod society, has a white tropical mess jacket that is cut very much like a PC. He has altered it so that the lapels and the cuffs are in the MacDougal tartan (his wife's tartan). He wears it with a 3-button vest in the Black Watch tartan. He wears this outfit with his MacLeod kilt, and knowing him, he probably rounds it off with cadadh hose made from yet another tartan. He can pull this off with elegance! But I don't think I'd reccomend it for everyone.

Aye,
Matt