Workaround: use the following approach:
private void commandBarDropDownList1_EnabledChanged(object sender, EventArgs e)
{
    if (this.commandBarDropDownList1.Enabled)
    {
        this.commandBarDropDownList1.DropDownListElement.TextBox.BackColor = Color.White;
    }
    else
    {
        this.commandBarDropDownList1.DropDownListElement.TextBox.TextBoxItem.BackColor = Color.FromArgb(210, 210, 210);
    }
}