Got something strange. Attaching a distance shader to a test surface layer turned the sky green, in preview and render. It had nothing to do with atmo or clouds, though. Default renderer, no funny stuff. Nothing to worry about, but just weird.
Was the test layer green?
The atmosphere is lit by the surface through GI (Enviro Light), so if that test layer got really bright in the distance it might do this.
Ah, GI, of course. The testlayer was the default purple, so the sky seems to have taken on its complement.