When you sit down to tie flies, aside from water clarity, what is the determining factor in choosing colors?
Several things determine which colors to use in a fly. If the water is stained, such as in the backcountry, flies that are predominately olive, light tan or brown, with gold flash, often work best. Much depends on the color of baitfish if that is what you are imitating. Bright, silvery baitfish are often best imitated with a white lower portion and a topping of pale blue, green or – my favorite – dark gray. With such baits I like to use silver rather than darker colored hooks with a combination of pearl and pink flash material.
If you are tying flies for bottom-feeding fish, the patterns should almost always match the bottom color. Two streamer flies that I am never without are all-white with a very short forward collar of red, and the one color I would use over all others: white or bright yellow lower portion with a topping of fluorescent chartreuse. During the past several decades, I have caught more fish on this color combination than all others. I also prefer to use gold flash with it.