ExcelAddress _formatRangeAddress = new ExcelAddress("J2:J"+(listaMargenes.Count+2));
   string _statement="IF(OFFSET(J3,0,-1)-J3>0,1,0)";
   var _cond4 = hoja.ConditionalFormatting.AddExpression(_formatRangeAddress);
  _cond4.Style.Fill.PatternType = OfficeOpenXml.Style.ExcelFillStyle.Solid;
  _cond4.Style.Fill.BackgroundColor.Color = System.Drawing.Color.Red;
  _cond4.Formula = _statement;

IF(OFFSET(J3,0,-1)-J3>0,1,0)

if(J3<0)

string _statement = "if(B3<0, 1)";