As someone who occasionally does some amateur drawing on the computer I often am upset about the quality of the colours of my display. Presets from the factory are nearly always total rubbish and the worst is: You don't believe it until you have two monitors side-by-side displaying the same colour completely different. Additionally my scanner also measures wrong colours, which I only noticed after having calibrated my display. So I decided to calibrate both devices.
I think this will be the most interesting part for most of you. Think about how much time you are spending in front of this thing every day. Shouldn't it then show you exactly the color it should?
The first thing you need in this part is some kind of calibration measurement hardware, in german called Kolorimeter. An interesting open source project is ColorHUG, nevertheless I decided to use a more common device, the Spyder4Express. If you have some experience with the ColorHUG please let me know, I would be very interested.
First of all you should turn off dynamic contrast in your monitor and wait approximately half an hour until your display has warmed up. Since most displays are way too bright you can also reduce the brightness to an amount you like. Keep in mind that you will lose a bit of brightness by calibrating.
Now that the hardware side is ready we can come to the interesting part: the software. I used the following workflow on an arch linux derivate with xfce desktop (Manjaro).
The process of calibration itself is pretty easy and self explanatory. Just start dispcalGUI, select the display you want to calibrate on the left, the default settings should be okay and click on "calibrate & profile". A little window appears where you can click the middle button to center it on your display. Now you have to position your colorimeter above this window. For this it has a little movable balance weight which you can place behind the monitor and then adjust the height of your colorimeter. After this step another window appears, where you have to use the settings of your monitor to adjust the white level and then finally "Continue on to calibration". In the middle of the screen is then shown a square that changes it color. This takes some time, you can get a coffee or some other drink.
When this step ran through you can find the finished icc-profile in ~/.local/share/dispcalGUI/storage/
For the calibration of the scanner you first need a so called target. There are some by experienced photo companies. Just search for "IT8 target", but these are all pretty expensive. Alternatively I can recommend you the site of IT8-Targets by Wolf Faust. Don't be scared by the ancient looking page, I just sent a letter with the money and very soon I got a letter back, containing my target and a CD containing the reference file.
When you got your target, you first have to clean the glass of your scanner a bit and then simply insert the image. Then you have to scan the target using exactly the software and the presets you will use later and save it as "scanner.tif".
Next you have to somehow get the reference file. Either you insert the cd and copy it from there or you download the corresponding file from the website and extract it.
Then use the command line:
scanin -v Dokumente/scanner.tif /usr/share/argyllcms/ref/it8.cht R130730/R130730.txt
If scanin says something about an incompatible tif file it may help to open it in gimp and save it again as tif-file.
The last step resulted in a ti3-file, which we can now use to make the icc-profile. For this you can use dispcalGUI: Open it and go to Options > Create profile from measurement data. There you have to select the scanner.ti3 file and then can save the finished icc-profile to a place of your choice.
Now you can use your scanner software with the same settings as above to scan an image and can then use a photo manipulation software like the gimp to add the icc-profile to correct the colors. In gimp you can make this by going to Image > Mode > Assign Color Profile. There you have to select the icc profile and gimp embeds it into the image. If you want to change the colors you can then use Image > Mode > Convert to Color Profile. There you select e.g. "sRGB" and gimp converts the colors.