You should write the code in row data bound event of the gridview. The following code will change the value of the corresponding row to yellow color.

protected void GridView_pm_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (!String.IsNullOrWhiteSpace(txt_search.Text))
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
var rowView = e.Row.DataItem as DataRowView;
if (String.Compare(rowView["MachineTypeNo"].ToString(), txt_search.Text,true) == 0)
{
e.Row.BackColor = System.Drawing.Color.Yellow;
}
else
{
e.Row.BackColor = System.Drawing.Color.White;
}
}
}
}

Thanks & Regards Taleeb (trystwithdotnet.blogspot.com)