Removing/renaming icc file in colors/in may lead to unpredictable results
Use a custom ICC profile by putting a proper file to ~/.config/darktable/colors/in. If a style is created with this ICC, or it is applied somewhere in the history stack, and afterwards the .icc file is removed, it seems that darktable does not check it. As a result, completely wrong colors are displayed without any warning. If one wants to fix the style or history stack (the local .xmp) containing the removed/renamed icc, one has to do it manually.