Another option is to handle the WM_ERASEBKGND and draw the dialog's background, eg:
Where hDlg is the handle of your dialog box. If you intend resizing your dialog you should handle the WM_SIZE and InvalidateRect(hDlg,0,1); to ensure proper drawing of the background. Adjust proportions to suit and it's probably better to initialise the background brushes when your app starts and delete them when your app finishes.
//create a black and a purple brush...
//...and use them to paint designated areas
//free gdi resources
Hope that helps.