Well, unfortunately, to keep the file size down they compress and if you're compressing an image that has 32-bit colours into something much smaller, as well as reducing it drastically in size, it "crushes" everything together in (un)expected ways. Hence the lime green on my daylight Endor matte and the blue haze on the nighttime one.
Still, you get the rest of the image clear enough and you can see what my intentions were. It's just annoying when you've done an image that's designed to stand up to detail even at 6k+ that you can't show it properly. Not that I want to be posting my full-res art for free, of course!