@Hesham: it is exactly that. The problem is that each time I close ODT, it seems to reset the color to "sea green". I don't manage to save the display setting of the track (colour, line width) and have to set it again each time.
@Uwe: thank you for the answer. However I 'm not sure to understand what you mean by the "Basemap window" (I can't find it in the View menu). When I restore my session, I have one "scene 1" window which is the 3D scene that is created by default when I created my project. If I add a Mapview window (Scene\ New Map View -- maybe the basemap you talk about ?) and if I add my track in this Map View tree, I can change the colour but I have the same problem if I close my session. At the reopening, the colour of the track is reset to "sea green" and the width to 3.
Maybe I don't understand the windows and data storage of ODT. I do everything from the 3D scene window as it is open by default. I don't understand the use of (and difference with) the map view.