Changing the opacity of a fit line

I'm trying to get my fit line to be the same opacity as the scatter points. How can I convert the line color from the way that I have it to the 4D color i.e [0 0 0 0.5] ?

 scatter(TDSad.Agemos,TDSad.(measureToAnalyze),'k','v','MarkerEdgeAlpha',0.5)%
    scatter(DSSad.Agemos,DSSad.(measureToAnalyze),'m','^','filled')%
   [sortedTD, sortItd] = sort(TDSad.Agemos);
    p1 = plot(sortedTD,TDall_yfit(sortItd),'LineStyle','--','Color','k','LineWidth', 2,);
    p2 = plot(DSSad.Agemos,DSall_yfit,'LineStyle','-','Color','m','LineWidth',2);

[Mathieu NOE]

hello

exactly the sme way as for scatter : , like

p1 = plot(1:10,1:10,'LineStyle','--','Color',[0 0 0 0.5])