I'm having an inverted colors issue too, in the main SL viewer, Catznip, and Kirstens Viewer S23. I tried to do a clean install of Firestorm to test it as well, but it's not even installing fully and I'm at a loss. But since it's happening in the main SL viewer then I have literally zero idea what might be causing it. Even my HUD colors are inverted. The funny thing is when I take a snapshot in-game and save to my computer, the picture looks perfectly fine. The inversion is only happening while I'm looking at it, but SL taking a snapshot of itself has the colors looking normal.
Screenshot of my viewer: (first attachment)
Snapshot using SL: (second attachment)
Definitely at a loss.