Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
This specimen was completely green when I collected it from among many others gathered on a midstream rock along with their nymphal shucks early in the morning. There was also a yellow one with them, which I assumed was a different species. Now that I've seen how this one started changing from green to yellow, I have to wonder if they weren't the same species and the yellow one was just older.
I believe so, but am unable to key it without being able to see hind wing venation. Do you have a photo which clearly displays hind wing venation?
If I were to guess at this point, it would be Haploperla brevis. However, if you have a photo clearly showing hind wing venation, it can accurately be keyed to (at least) genus.
This certainly looks like Chloroperlidae to me, but I've long been curious about the species. The late May/early June emergence of this species has always been heavy in the Brodhead drainage. The fish largely ignore them when the olive morning dun hatch is in progress, but I've had some success imitating them on early June mornings after the main Drunella hatch ends. I like Roger's Haploperla brevis guess, mostly because it was on the short list of species I was considering as candidates. I'd love to know the species ID for sure, but I doubt that the pictures provide the means to take this specimen to that level.