Everything works as intended in the editor and when running as a standalone process with iOS Metal preview. The compiled thing running on an actual iPhone, however, doesn’t have the correction. What could be the possible reasons?
You have to remove:
// Don't do color correction on iOS or Android, we don't have the GPU overhead for it. #if !(PLATFORM_IOS || PLATFORM_ANDROID)
Since making this question, I just made a palette system: In blueprints, I store the colour values for things that are important to distinguish by colour - depending on settings in-game, they change to pre-selected values. This could also be done with global material parameters, even with static materials. Way better than a fullscreen effect that applies to everything, and introduces more control and better results.