how to get selectedvalue of dropdown populated frm database

Question

I have a dropdownlist box
<asp:DropDownList ID="drpAvailableColours" runat="server" AutoPostBack="True" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged" EnableViewState="true" >
</asp:DropDownList>
I am populating the database using the code---
DataSet dsColours = new DataSet();


string strColourQuery = "Select ProductName,ColourCode from ProductDet where ProductId in (select ProductId from ProductMaster where CatId = '1' and SubCatId='1' and BrandId='2' and SubBrandId='7')";


dsColours = db.GetTableDefinedDataSet(strColourQuery, "ProductColours");


if (dsColours.Tables["ProductColours"].Rows.Count > 0)
{
var query = dsColours.Tables["ProductColours"].AsEnumerable()


.GroupBy(x => x.Field<string>("ProductName").ToString())


.Select(x => x.First());






foreach (DataRow DistinctProducts in query)
{






drpAvailableColours.Items.Add(new ListItem(DistinctProducts["ProductName"].ToString(), DistinctProducts["ColourCode"].ToString()));






}


}
else
{
//postback
Label1.Text = drpAvailableColours.SelectedValue.ToString();
}




But I don't get the selected value from the dropdownlist in the label1
Also DropDownList1_SelectedIndexChanged doesn't trigger when I select from dropdownlist