Since the above answer about waistcoat v. vest has already been given...

I think nearly any color looks fine with Blackwatch. It's a pretty neutral tartan from that perspective (again, to me). I would let your personal comfort guide the color decision. For example, red would be a great contrasting color and look fantastic to some. On the other hand, it may be a bit too loud a contrast for others.

How's that for a non-answer?

I personally like less flamboyant colors on myself. So I would go toward browns and/or grays. If I did go with a red, it would be more maroon--a deeper color and less "bright".

My 2 cents...