In the Little Red Book of Fly Fishing, Kirk Deeter has a writeup on the color purple. In this writeup he writes "One theory is that purple catches fishes' eyes better than other shades. Among trout, for example, we know, according to Dr. Robert Behnke, author of Trout and Salmon in North America, that the cones in the retinas of trout eyes are more receptive to shades on the blue side of the spectrum." He goes on to say that "when they're on a hatch, matching size and color is key, but when they are just opportunity feeding, gaudy is better". I have actually used purple PT's on my home stream with good success. It's not my "go to" color, but I have used it as a second or third choice. On one particular day I nailed two 20-inch trout using the fly so I can say that I am a believer in purple flies.
I think Behnke has it right. Most of the time, trout are not feeding selectively and are sampling the drift. Since fish don't have hands, the only way they can tell if something food or not food is to "sample" it.
There is nothing natural that looks like a Royal Wulff but a Royal Wulff is the most successful dry fly attractor.