Well from your markup I see it is lacking the event:
<asp:dropdownlist id="combobox" runat="server" autopostback="true" enableviewstate="true" onselectedindexchanged="combobox_SelectedIndexChanged" xmlns:asp="#unknown">
<asp:listitem value="White" selected="True">White</asp:listitem>
<asp:listitem value="Black">Black</asp:listitem>
<asp:listitem value="Blue">Blue</asp:listitem>
</asp:dropdownlist>
In your code behind:
protected void combobox_SelectedIndexChanged(object sender, EventArgs e)
{
}
This should work.
Good luck,
OI