Change Background Color of Gridview inside AJAX ModalPopupExtender in ASP.Net

You can use CSS to change the background color of GridView.

<style type="text/css">
    .table
    {
        background-color: Silver;
    }
</style>

Apply this style in the GridView

<asp:GridView ID="GridView1" CssClass="table" runat="server"  
    AutoGenerateColumns="False" CellPadding="5">
    <Columns>
        <asp:BoundField DataField="ProductID" SortExpression="ProductID" HeaderText="Product ID"
            HeaderStyle-HorizontalAlign="Left" />
        <asp:BoundField DataField="ProductName" SortExpression="productname" HeaderText="Product"
            HeaderStyle-HorizontalAlign="Left" />
        <asp:BoundField DataField="UnitPrice" SortExpression="unitprice" HeaderText="Price"
            HeaderStyle-HorizontalAlign="Left" />
    </Columns>
</asp:GridView>

Image:

Thank You.