How to set cell color programmatically epplus?

Check your line:

if (dtdata.Rows[4].ToString() == "Annual Leave")

If it is a standard .net table wouldnt .ToString() evaluate to "System.Data.DataRow"? Also ws.Cells["E1"] will need to be adjusted for each cell after looping through the row count (basically what krillgar was saying).

Something like that:

[TestMethod]
public void Cell_Color_Background_Test()
{
    //http://stackoverflow.com/questions/28679602/how-to-set-cell-color-programmatically-epplus

    //Throw in some data
    var dtdata = new DataTable("tblData");
    dtdata.Columns.Add(new DataColumn("Col1", typeof(string)));
    dtdata.Columns.Add(new DataColumn("Col2", typeof(int)));
    dtdata.Columns.Add(new DataColumn("Col3", typeof(int)));

    for (var i = 0; i < 20; i++)
    {
        var row = dtdata.NewRow();
        row["Col1"] = "Available";
        row["Col2"] = i * 10;
        row["Col3"] = i * 100;
        dtdata.Rows.Add(row);
    }
    //throw in one cell that triggers
    dtdata.Rows[10]["Col1"] = "Annual leave";

    var existingFile = new FileInfo(@"c:\temp\temp.xlsx");
    if (existingFile.Exists)
        existingFile.Delete();

    using (var pck = new ExcelPackage(existingFile))
    {
        //Using EPPLUS to export Spreadsheets
        var ws = pck.Workbook.Worksheets.Add("Availability list");

        ws.Cells["A1"].LoadFromDataTable(dtdata, true);

        ws.Cells["A1:G1"].Style.Font.Bold = true;
        ws.Cells["A1:G1"].Style.Font.UnderLine = true;

        //change cell color depending on the text input from stored proc?
        //if (dtdata.Rows[4].ToString() == "Annual Leave")
        for (var i = 0; i < dtdata.Rows.Count; i++)
        {
            if (dtdata.Rows[i]["Col1"].ToString() == "Annual leave")
            {
                ws.Cells[i + 1, 1].Style.Fill.PatternType = OfficeOpenXml.Style.ExcelFillStyle.Solid;
                ws.Cells[i + 1, 1].Style.Fill.BackgroundColor.SetColor(System.Drawing.Color.LightYellow);
            }
        }

        pck.Save();
    }