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.