To effectively fish a dry fly you have to be able to readily see it so that you can see if a fish takes it as well as monitor your cast for accuracy and drag. All of my dry flies have white poly wings or white poly overwings. I feel that when trout are feeding on mayflies they key on the length of the body as well as the overall size and color of the fly. The color should be close to the natural but does not need to be exact. As a mayfly floats down river the fish are looking up at it from the underside of the insect and the vertically position wing is only vaguely visible to the trout if at all. A natural dun colored wing looks good to the fly fisher but I think the color of the wing is irrelevant to the fish. as is the tail. Many trout are caught on dun patterns with pink or chartreuse wings. The Vis-A-Dun is tied with the hackle clipped under the hook shank so that the fish gets a good look at the body and has a white wing so the the angler gets a good look at the fly.
When fishing a mayfly hatch I always fish a Vis-A-Dun with a Barr Emerger dropper which match the hatching mayfly and use a combinations of techniques. First I dead drift the combo, and if no response I add a couple twitches to the flies just before they get to the fish which can trigger a take. If the fish is below me I will dead drift, twitch and if no response I will swing the flies into the rising trout and stop the drift so the emerger rises up as it gets to the fish. I feel trout, especially heavily pressured fish scrutinize surface patterns before refusing or taking them whereas they will often take the sunk emerger with no hesitation.