If you really must do this (and I agree with Nigel that this is not a good UI choice) then you must use C API calls respond to message and repaint the non-client area of the title bar.

This can be done but is a LOT of effort and includes some nice gotcha's like not monitoring the WM_WININICHANGED message (User changes font size etc) and having to paint the control box, minimise, maximise/restore box.