To keep things simple, for the System Preferences > Displays > Color tab I suggest ticking the "Show profiles for this display only" checkbox. That will limit the choices to just the display profile(s) suitable for your iMac's screen, which most likely will be just the "iMac" choice unless you have created a calibrated profile for it.
While you are at it, it is generally a good idea to set the "Night Shift" schedule to off. That's because this will avoid misleading color shifts while you are working, & also because the benefits of reducing the blueness of the display to improve sleep are based on an iffy premise about how that affects the color spectrum of the display vs. the spectrum of natural sunlight.
kkiamhere got a reaction from Oval in Why Affinity Designer & Photo is showing different colour hue?