Thank you, dmpr ! That was the missing link :)
Unfortunately it does not work yet, but maybe I have to dig deeper....
I changed the file topo.render.xml. There is a constant defining the color of a mountain path: " <renderingConstant name="sacScaleColorT4" value="#6000ff"/>"
I changed the hex code of the color to something like pink (#f900f9). Then I used iCloud and asked OsmAnd Maps to open it (via share menu). No change in the line color so far. But maybe I just don't understand the rendering-xml good enough.
Any hints welcome .... Thanks again,
Jo.