I guess I tie on a black fly -- with maybe some other color mixed in -- in off-colored water because it's a confidence issue.
Geez, I sound like one of those bass pros on the TV fishing shows.
Anyway, I can see the fly better, so I feel like the fish can see it better. I'm no expert on winter steelhead -- I flog the water to a froth and don't catch too many -- but I always, always, always tie on a black fly in Oregon's Deschutes River when the White River pumps color into the stream. I like a combination of black and bright yellow or bright red.
This may be slightly off topic, but my catch rates for bass in colored water go way up when I tie on a black woolly bugger....