But at the moment i do not know how to colorize the circle -
note: i do not want ot fill the are - within the cirle - no: i only want to colize the border of the circle.
how to do that - how can i do that!? - How to achieve this!?
Make a circular selection, you already know how to do that.
In the menu “Select” click on “Selection Editor”. Selection Editor tab will appear to the right of the canvas.
In the Selection Editor tab, click on the button “Paint along the selection outline” (see screenshot below).
A window titled “Stroke Selection” will appear, you can select the colour, paint tool, line width, etc… Experiment with this a bit to get the result you like.