displaying a Message when colour is not chosen from the combo box

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MFC

Hi ,
Im doing a program for changing the colour of controls selected from the combo box. my doubt is , when push button is clicked without selecting a colour from combo a message must appear " select a colour". im not getting the pop up message,,,if i get the pop up message the control colour is not changing, I dono wat mistake i had done .pls correct me ,,,
My coding

void CComboTestDlg::OnButton1()
{
    int index = m_ColorCombo.GetCurSel();
if(index != CB_ERR)
{
        m_brush.DeleteObject();
        switch(index)
        {
        case 0:
        m_brush.CreateSolidBrush(RGB(0, 240, 120));
        break;
        case 1:
        m_brush.CreateSolidBrush(RGB(128, 0, 240));
        break;
        case 2:
        m_brush.CreateSolidBrush(RGB(0, 128, 0));
        break;
        case 3:
        m_brush.CreateSolidBrush(RGB(128, 128, 0));
        break;
        case 4:
        m_brush.CreateSolidBrush(RGB(0, 0, 128));
        break;
        default:
            m_brush.CreateSolidBrush(::GetSysColor(COLOR_WINDOWTEXT));
            break;
    }
        Invalidate(TRUE);
}
else
        {
        AfxMessageBox("Select A colour");
        }
}

thank you