As stated, you can wear about any color. Pick what you like.

But since you asked: I'd wear lovat green with red flashes, just like the Blackwatch did and God intended. Looks great, understated but tasteful. In fact I often wear a pair I bought from the Black Watch Regimental store (the one in Scotland which may not exist anymore). I also have two other pair, one from Lewis, which I like even better. I wear them with my Robertson Hunting (which is basically a Black Watch with some red and blue stripes added) as well as just about any other kilt I own.

But then there is the really cool claret colored pair knitted custom by piperdbh. With navy flashes, they're really slick!