I'm seeing it happen to me lately. It seems some of my art is getting uploaded less bright compared to the original for some reason. I have no idea what's causing it either. It happens here and twitter and I have no idea if its me or something changed with the uploading system.
Doki-Draws
I'm late on this, but could it be because the image is converted to Webp on Newgrounds?
I know for sure that Twitter compression is bad, and can mess with colors.
plankboy
I learned what caused it. It was Google Chrome.