Hi,
the LineSpec argument is limited to some specific line style characters plus color short name characters listed in the documentation of yline.
Use e.g.
yl1=yline(-0.25,'-','Freibord 75% ausgenutzt','HandleVisibility','off',...
'Color','#D95319');
or
yl1.Color = [0.8500 0.3250 0.0980];