After installing the latest K-Lite Codec pack and NVIDIA drivers I found that some video files had weird colours.
After some futzing around I found the issue.
Go to the NVIDIA tray icon, select color correction, at ‘Apply color changes to’ select ‘All’. Then at ‘Color profile’ select ‘Advanced mode’ and then ‘Restore Defaults’, select ‘Apply’. This fixed my colour problem. Apparently the new NVIDIA drivers/control panel do something with the gamma, which isn’t shown in the panel.
Caused me a few days of head breaking, especially since it only was for some codecs/applications.