I tried setting all related properties ExcelLineChartSerie has to offer, but still cannot set or change the color of the Excel marker from default ugly blue.
ExcelChart ec = ws.Drawings.AddChart("LineChart01", eChartType.LineMarkers);
var rangeX = ws.Cells["A2:A11"]; // X-Axis
var range1 = ws.Cells["B2:B11"]; // 1st LineSerie
ExcelLineChartSerie serie1 = (ExcelLineChartSerie)ec.Series.Add(range1, rangeX);
serie1.MarkerLineColor = System.Drawing.Color.Gray;
serie1.MarkerSize = 10;
serie1.Fill.Color = System.Drawing.Color.Gray;
serie1.LineColor = System.Drawing.Color.Gray;
serie1.Border.LineStyle = eLineStyle.Solid;