曲线救国的办法，重写OnEnabledChanged方法，Enabled=false时，将DropDownStyle属性修改为DropDownList，Enabled=true时恢复原来的DropDownStyle属性，这样在应用时就可以通过设置BackColor来改变下拉框的背景色了。

protected string style = "DropDown";
protected Color color = Color.Black;
public MyComboBox()
{
    InitializeComponent();
    this.DrawMode = DrawMode.OwnerDrawVariable;
    this.MaxDropDownItems = 20;
    if (this.DropDownStyle == ComboBoxStyle.Simple)
        style = "Simple";
    else
        if (this.DropDownStyle == ComboBoxStyle.DropDown)
            style = "DropDown";
        else
            if (this.DropDownStyle == ComboBoxStyle.DropDownList)
                 style = "DropDownList";
    color = this.ForeColor;
}

protected override void OnEnabledChanged(EventArgs e)
{
    base.OnEnabledChanged(e);
    if (!this.Enabled)
    {
        this.DropDownStyle = ComboBoxStyle.DropDownList;
        this.ForeColor = Color.FromArgb(109, 109, 109);
    }
    else
    {
        switch (style)
        {
            case "simple": this.DropDownStyle = ComboBoxStyle.DropDownList; break;
            case "DropDown": this.DropDownStyle = ComboBoxStyle.DropDown; break;
            case "DropDownList": this.DropDownStyle = ComboBoxStyle.DropDownList; break;
        }
        this.ForeColor = color;
    }
}