You could implement a reaction on WM_ERASEBKGND for your bar :) :
...
BEGIN_MESSAGE_MAP(CYourBar, CYouBarBase)
ON_WM_ERASEBKGND()
...
END_MESSAGE_MAP()
BOOL CYourBar::OnEraseBkgnd(CDC* pDC)
{
CRect cClientRect;
GetClientRect(cClientRect);
pDC->FillSolidRect(cClientRect, RGB(0, 128, 0));
return FALSE;
}
...