Together with Kolor Manager & Oyranos, KWin supports full screen colour correction (not only calibration, but also colour space transformation using Xcm specification).

Once Colour Correction of KWin is turned off, the kded module kolorserver should be turned off as well. When kolorserver continues to run, applications not supporting Xcm (almost all) continue to believe that monitor's profile is sRGB and since KWin does not do Color Correction (from sRGB to monitor's profile) anymore, the colors are displayed wrongly by the applications (GIMP, darktable, EoG, ...).

Reproducible: Always

Steps to Reproduce:
1. Install Oyranos + Kolor Manager.
2. Set KWin to use OpenGL and activate Colour Correction in Special Effects/Advanced.
3. Check that kolorserver is running (in Services) or via D-Bus.
2. Check on CLI that the Xcm (X color management specification proposal) X properties were set by running "xprop -root | grep ICC | less", in particular there should be:
_ICC_DEVICE_PROFILE (the actual monitor's ICC profile)
_ICC_PROFILE (in Xcm this holds the sRGB colour space, this property is defined by XICC spec).
and some others.
3. The above steps confirm that full screen colour correction is working.
4. Now, turn off Colour Correction in Special effects.
Actual Results:  
Colour Server (kolorserver in D-Bus) continues to run and all the Xcm X properties are unchanged.

Expected Results:  
kolorserver should stop.

You can stop kolorserver manually via D-Bus (just unload the module in kded - I used qdbusviewer-qt4 to do that in GUI).
Once stopped, the Xcm X properties are removed and XICC X property (_ICC_PROFILE) is properly set to the actual monitor's profile instead of sRGB colour space.