No way to apply selective B&W adjustment

Hi.

Applying B&W adjustment selectively, either positively or negatively, with a brush doesn't seem to work, at least it does not show on screen. If an image is turned B&W, it shows on screen. Then, if you try to apply the effect with a brush, the entire image switches back to its color version, but brushing in the effect will not be seen. Or else, if you turn an entire image to B&W, it will show on screen, but you will see no effect if you use a negative brush to insert color back to some areas.

Aperture 3.2 and Mac Os 10.7.2