Match the ???

Adult Mayfly Dun Trout Fly Fishing Murray's Fly Shop VA
By matching the color and size of the natural insects you will do well.

Personally I enjoy entomology. I found that working with Art Flick in the sixties in order to identify the aquatic insects in Virginia very rewarding and it was the beginning of a very special friendship.
We now have many excellent books on aquatic entomology so if you enjoy this I believe you will find it very rewarding.
However, one of the greatest trout anglers and finest gentleman I have ever known used a different approach.  He was not into technical entomology, but he did not seem to need it. When he saw large tan natural aquatic insects on the stream he would take an artificial fly from his box which matched it and catch many trout. If he saw small yellow natural insects on the stream he would go to his fly box and match it with great success.  His name was Jack Sperry and he was a true master angler.