<%@ Control Language="C#" ClassName="MyUserCtrl" %>
<script language="C#" runat="server">
   public event EventHandler WhiteSelected;
   void Btn_Click(Object sender, EventArgs e) {
      if ( Color.SelectedItem.Value == "white" )
         WhiteSelected(this, EventArgs.Empty);
   }
</script>
Color : <asp:dropdownlist id="Color" runat="server">
            <asp:listitem>white</asp:listitem>
            <asp:listitem>black</asp:listitem>
         </asp:dropdownlist>
<asp:button id="Button1" text="Submit" OnClick="Btn_Click" runat="server"/>