Thank you!
I did
recommendationOneSeries.Points.Clear();
recommendationOneSeries.Area.FillFormat.FillType = Excel.Cells.Drawing.FillType.Automatic;
recommendationOneSeries.Area.ForegroundColor = System.Drawing.Color.FromArgb(64,196,124);
recommendationOneSeries.Area.Transparency = transparency;
recommendationOneSeries.Area.FillFormat.FillType = Excel.Cells.Drawing.FillType.Solid;
,
and it seems to be working fine now.