Do you want this ?:
Js Code:

    <script type="text/javascript">
        function setForColor() {

            var grid = ISGetObject("WebGrid1");

            for (var i = 0; i < grid.TotalRows; i++) {

                var row = grid.RootTable.GetRow(i);
                var rowElement = row.GetElement();
                rowElement.style.color = "red";
           
            }      
   
        }
    </script>

C# code:

//Set webgrid BackColor 
      protected void Button1_Click(object sender, EventArgs e)
    {
        WebGrid1.LayoutSettings.AlternatingColors = false;
        WebGrid1.RootTable.RowStyle.BackColor = Color.Red;
    }

Hello,

To change background color via Server-side, you can use OnInitializeRow Server-side event such as following:

protected void WebGrid1_InitializeRow(object sender, ISNet.WebUI.WebGrid.RowEventArgs e)
{
   e.Row.Style.BackColor = System.Drawing.Color.Red;
}

To change background color via Client-side, you might want to refer to javascript code given by Frank Bruce. The code has been successfully tested and it will change the text color in your WebGrid.

Thank you.