code

IMapFeatureLayer layer = map1.Layers.Add(fs);
LineSymbolizer line = new LineSymbolizer();
line.SetWidth(20);
line.SetFillColor(Color.Blue);
line.SetOutline(Color.Red,5);
layer.Symbolizer = line; 

pruduces blue line with BLACK border. but not the red one.
and code
LineSymbolizer line = new LineSymbolizer(Color.Blue,Color.Blue,20,DashStyle.Solid,LineCap.Flat);
layer.Symbolizer = line;
produces line with additional white border (see attachment)
 
so I can't create custom line with my fill and border colors in normal way