无法更改下拉框的背景颜色

Question

我正在做ASP.NET应用程序，我有一个面板控件，我想将一个子控件的背景色更改为黄色，我检查了dropdownlist的值，如果发现它等于某个条件，那么我将更改dropdownlist背景颜色变为黄色。但是，我看不到控件的颜色已更改。有什么问题吗？

        private void MarkUnknownClassColumn()
        {
            for (int i = 0; i < uiPanelControls.Controls.Count; i++)
            {
                WebControls.UsageControl2 ctrl = uiPanelControls.Controls[i] as WebControls.UsageControl2;
                if (ctrl != null && ctrl.Visible)
                {
                    for (int j = 0; j < ctrl.Controls.Count; j++)
                    {
                        Panel pnl = ctrl.Controls[j] as Panel;
                        if (pnl != null)
                        {
                            for (int k = 0; k < pnl.Controls.Count; k++)
                            {
                                WebControls.UsageControl2SubControl1 ctrl2 = pnl.Controls[k] as WebControls.UsageControl2SubControl1;
                                if (ctrl2 != null && ctrl2.Visible)
                                {

                                    RadDropDownList uiClasses = ctrl2.FindControl("uiClasses") as RadDropDownList;
                                    HtmlGenericControl uiColumnUnit = ctrl2.FindControl("uiColumnUnit") as HtmlGenericControl;
                                    RadNumericTextBox uiDistance = ctrl2.FindControl("uiDistance") as RadNumericTextBox;
                                    RadDropDownList uiUnit = ctrl2.FindControl("uiUnit") as RadDropDownList;
                                    if (uiClasses != null && uiClasses.SelectedItem.Text == "Unknown")
                                    {
                                        uiClasses.BackColor = System.Drawing.Color.Yellow;
                                    }
                                }
                            } // for (int k = 0...
                        }
                    } // end for (int j = 0...
                }
            } // end for (int i = 0...
        }