I'm with Salmo g. - my experience too is that the fish can see much better in off-color water than we think I have caught fish when the water was so dirty that I could not see the stream bottom when the tops of my boot toes just awashed (less than 4 inches of vis).

Generally I don't think the fly color is extremely important though black is as verstible as any being a go to color in both very clear or dirty water. Just generally small profiles in clear water and big in dirty water. Typically when faced with visibilities of 1 foot or less I fish nothing but dark flies (black with maybe a hint of another color). With 18 inches or better typically change things up as the mood strikes me.

In high and dirty waters I think the most important factor in success is where you fish. On dropping water I focus on moving fish putting my fly through those traveling lanes which can be pretty shallow. In such conditions I have caught more than a few fish in less than 18 inches of water. The location of those fish are very predictable and in such circumstances one can fish very efficiently.

Tight lines
Curt