Chart Line Colors

I ran the macro recorder to try and answer your Q. The ColorIndex property seems the probable suspect

    ActiveChart.SeriesCollection(1).Select
    With Selection.Border
        .ColorIndex = 51
        .Weight = xlThin
        .LineStyle = xlContinuous
    End With
    With Selection
        .MarkerBackgroundColorIndex = xlNone
        .MarkerForegroundColorIndex = xlNone
        .MarkerStyle = xlNone
        .Smooth = False
        .MarkerSize = 17
        .Shadow = False
    End With