Yep it's a weird known issue, it's even worse on some other levels. Inferno, for instance, has a neon green skybox.
If you set up the game to run through dgVoodoo it fixes this issue and all the skyboxes appear as normal. I can't post links here but do a forum search for "Advanced Tips and Improvements" and you'll see the links and setup instructions.
Post edited May 09, 2018 by rglennbarker