To calibrate a monitor and generate a color profile for it on Linux, you need two things: a colorimeter and color profiling software. High-quality professional colorimeters tend to be rather expensive, but you can use the excellent open source ColorHUG device instead. When it comes to calibration and profiling, the displaycalGUI software is the perfect software for the job. It provides a graphical user interface to the display calibration and profiling tools of the Argyll CMS open source color management system. The latter is available in the software repositories of most mainstream Linux distributions, so you can install it using your distro’s package manager. On Debian and Ubuntu-based distributions, this can be done by running the apt-get install argyll command as root. Grab then the packaged version of dispcalGUI from the project’s website and install it on your system. Connect ColorHUG (or any other supported colorimeter) to your machine and launch dispcalGUI.
Press the Refresh button to make dispcalGUI detect the connected colorimeter (it should appear in the Instrument/Port section). If you are using a multi-display setup, make sure that the correct monitor is selected in the Display device section. Select then Photo from the Settings drop-down list. Press then the Calibrate & Profile button and follow the instructions to calibrate the monitor and generate its color profile. Once the color profile has been generated, dispcalGUI prompts you to install the profile and enable it during boot.
Launch then digiKam, choose Settings → Configure digiKam, and switch to the Color Management section. Click on the Profiles tab, select the generated profile from the Monitor profile drop-down list, and press OK to save the settings.