Maybe it's late, but I have a puzzle I just can't seem to figure out. I am trying to create a very slight white opaque animation for the bottom left of a TV show. I want to place a shadow on it but because the white is transparent everything seems to come out grey. Does anyone have any ideas on how to create an all white semi-transparent graphic and add a drop shadow while keeping it looking OK?
Rather than using a layer style drop shadow, duplicate the layer twice and turn them both black (same shape, just black). Move one down and right a few pixels. That's your shadow. The other use as an inverted matte for the offset one so that the only thing that is visible is the offset sliver where the two layers do not intersect. You might need to turn the masking layer white, but the concept is the same.
That way the white opaque shows the background, but still has a black rim to act as a shadow. You can of course make the black rim layer semi transparent as desired.