Use openxml to create the new cellformat and apply it to cell. Use openxml to modify the cellformat of cell.

gistfile1.cs

                    var styleSheet = document.WorkbookPart.WorkbookStylesPart.Stylesheet;
                    Fill fill = new Fill() { PatternFill = new PatternFill { ForegroundColor = new ForegroundColor { Rgb = fgcolor }, PatternType = PatternValues.Solid } };
                    styleSheet.Fills.Append(fill);
                    //or use the method Count(),not need to plus 1,but slowly.
                    var fid = styleSheet.Fills.Count++;

                    CellFormat cellFormat = null;
                    if (theCell.StyleIndex.HasValue)
                    {
                        var originalCellFormat = document.WorkbookPart.WorkbookStylesPart.Stylesheet.CellFormats.ToList()[(int)theCell.StyleIndex.Value] as CellFormat;
                        //copy the original cellformat data to the new cellformat
                        if (originalCellFormat != null)
                        {
                            cellFormat = new CellFormat(originalCellFormat.OuterXml);
                        }
                        else
                        {
                            cellFormat = new CellFormat();
                        }
                    }
                    else
                    { 
                        cellFormat = new CellFormat();
                    }
                    cellFormat.FillId = (UInt32)fid;
                    styleSheet.CellFormats.Append(cellFormat);
                    var theStyleIndex = styleSheet.CellFormats.Count++;
                    theCell.StyleIndex = new UInt32Value { Value = (UInt32)theStyleIndex };
                    document.WorkbookPart.WorkbookStylesPart.Stylesheet.Save();

gistfile2.cs

                    var theStyleIndex = theCell.StyleIndex;
                    var fillId = ((document.WorkbookPart.WorkbookStylesPart.Stylesheet.CellFormats.ToList()[(int)theStyleIndex.Value]) as CellFormat).FillId.Value;
                    var fgcolorobj = document.WorkbookPart.WorkbookStylesPart.Stylesheet.Fills.ToList()[(int)fillId].Descendants<PatternFill>().First();
                    if (fgcolorobj != null)
                    {
                        if (fgcolorobj.ForegroundColor != null)
                        {
                            fgcolorobj.ForegroundColor.Rgb = new HexBinaryValue { Value = fgcolor };
                        }
                        else
                        {
                            fgcolorobj.ForegroundColor = new ForegroundColor { Rgb = fgcolor };
                        }
                    }
                    document.WorkbookPart.WorkbookStylesPart.Stylesheet.Save();