If you use HOF Suite to make hof files, there is a built-in configuration panel to choose text color and background color for the line number. You can access it by clicking the gear icon on the right.
Yeah, I actually worked out this one already. I found a great video on YouTube that explained it very well. You actually have to paint those colored displays on Paint.net and you put the ready ones to the folder of Krueger 230x32 in the Anzeigen folder that is located in the Vehicles folder.
If you search on Google: "omsi 2 girouette" you'll find the video I was talking about.