Missing support for color management of widgets

Issue: QT appears not to have support for applying color management to widgets, at least on Linux.

Details: KDE has an issue where the colors used by the operating system are not properly color managed. On Linux under X11, color management is usually handled by colord. I have a profile installed for my screen in colord, and it's used by all color managed applications as expected, but the colors used by the "Plasma" theme in KDE applications are not color managed. KDE bug report here.

KDE devs indicated that the issue with implementing this is that QT does not provide support for color managing the UI elements, and asked that I file a bug here.

Common consumer screens have been getting wider gamuts over the last few years, and so this can matter quite a lot, as colors chosen by KDE's visual design group will vary significantly from user to user as a result. As an example, please open this screenshot in a color managed image viewer. The color on the left is what I see in KDE. The color on the right is the actual developer intended color as shown in a color managed viewer. As you can see, the result is very different, and arguably the version that isn't color managed looks quite bad (if you have a screen with a large enough gamut to see the difference).

Apologies if I've made any mistakes in how this bug was reported, this is my first issue on this tracker.