This line:
B2series.Points(c).Color = (int)Excel.XlRgbColor.rgbRed
"suggest" :) that you want to change the color of Point of Chart.
then...(VBA version will look like)
B2series.Points(PointNumber).MarkerBackgroundColor = RGB(0,255,0)
B2series.Points(PointNumber).MarkerForegroundColor = RGB(255,0,0)
B2series.Points(PointNumber).MarkerStyle = xlMarkerStyleCircle