Cannot change some markers in chart

I want to change the color of some markers in a serie. With the implementation of the following code it doesn't work. 

Any idea how to implement this in a correct way?

Chart.BeginUpdate;

with Chart.Panes[0].Series.Add do 

 begin 

 ClearPoints; 

 ChartType := ctMarkers; 

 ShowValue := False;

 

Marker.MarkerType := mCircle; Marker.MarkerSize := 13; Marker.MarkerColor := clGreen; Marker.MarkerColorTo := Lighter(clGreen, 50); Marker.MarkerLineColor := clGreen; Marker.GradientType := gtForwardDiagonal;

// add all points

for i := 0 to _series.Count - 1 do

       begin

          ecoGraphSerie := TEcoGraphSerie(_series);