The overcast was intentional, although I might've exaggerated it.
Yeah, I jumped the gun on the 'Union' part, I downloaded the image labeled 'Dead on Battlefield at 1st Bull Run' or something similar, and I mislabeled it - Not just here, but elsewhere, lol. Yes, it's definitely not from 1st Bull Run, I argued with a user on Reddit and the first verfiable 'death' photograph I was able to find was definitely from Antietam, as you mention. I don't know if it is from Antietam aswell, don't know who'd be able to shed some light on that.
What I do know, though, is that I couldn't get the grass to look right any other way, and you know this - It looked too 'off'.
No need to apologize for the attribution. I have had to re-label many of my images in the past when corrected information comes to light.
As for the color of the grass, I know exactly what you mean, sometimes it seems that it just does NOT want to hold the color that you know it should have. What a pain. I have had the same thing with the green on cannon carriages.