TCustom Control transparency problem

hi to all,
            I have derived a component from TCustomControl, i load some
transparent bmp and i draw on it.
My problem is that the images are really transparent but the canvas color of
my object is the same of the parent control even if it should have to be

i make an example because may be it is easyer to understand:

I have a gray form; on it a white TShape and a black shape
if a place my object on the two shape, and i draw a circle on it, the canvas
background isn't black and white but only a gray rectangle as my form

does any one know why?