MFC radio buttons control

[grayfox]

hello,
I have an exercise to build a simple GUI in MFC. I need to make few radio buttons which were change a background of my rectangle. Here is my code :

void CDialogRisanje::OnPaint()
{
	CPaintDC dc(this); // device context for painting
	// TODO: Add your message handler code here
	// Do not call CDialog::OnPaint() for painting messages
	
	CRect r;
	GetClientRect(r);
	CBrush bela(RGB(255,255,255));
	dc.SelectObject(&bela);
	dc.Rectangle(r.Width()/2-320,r.Height()/2-200,r.Width()/2+130,r.Height()/2+200);

}

void CDialogRisanje::OnBnClickedRadio1()
{
	

}

Problem is that I don't know how to change a background with a press on radio button. Tnx for help.