How to set color or background with "excelpackage"

Try something along these lines (Taken from the EPPlus sample files provided):

using (var range = worksheet.Cells[1, 1, 1, 5]) 
    {
        range.Style.Fill.PatternType = ExcelFillStyle.Solid;
        range.Style.Fill.BackgroundColor.SetColor(Color.DarkBlue);
    }