Make sure RGX is off at the URL opera://settings/lucidMode.
Check at the URL opera://mods to see if you have any mods that support shaders. If so, turn shaders off at the URL opera://settings/shaders_settings.
See https://www.reddit.com/r/operabrowser/wiki/opera/dark_mode_settings/. Maybe dark mode is on for all sites.
It could be an extension. Disable your extensions one by one at the URL opera://extensions to see if one is causing it.
If you goto the URL chrome://flags and search for color, you'll see the "Forced colors" and "Force color profile" settings. You can mess with those if needed. Put them back to default if they don't help though.
You can goto the URL opera://settings/system and disable hardware acceleration to test if that fixes it.
You can test things in a private window to see if the colors are fine there or not.
