Photoshop :: Add Color Cast To Grey?

I'm trying to add a color cast to a grey, contoured button shape (defined by a vector mask), by overlaying it with an adjustment layer with a solid green fill and setting the blend mode to "multiply."
 
The shape layer has transparent pixels around the perimeter, so I would not expect a "multiply" operation to result in visible green there.  But it does.  That's no good.  I need to export this image to a PNG with the transparency intact.
 
I can see that the green adjustment layer has a "Color Fill 1 Mask" channel, but copying the shape layer and pasting it into that mask channel doesn't seem to work. The display shows a very thin outline of the vector shape, but I can't appear to do anything with it.