Photoshop multiplies by zero, gets non-zero

Question

Hi all.
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 help.  The display shows a very thin outline of the vector shape, but I can't appear to do anything with it.
What's the proper way to accomplish my goal here?
Thanks!

Answer 1

"When you set the Blend mode to Multiply, transparency is not multipled."
Exactly.  I wouldn't expect it to be.  As you say, the COLOR is supposed to be multiplied, and there isn't any color in the transparent region to multiply by.  Reasonably expected result = zero.  It would make sense to offer a toggle to treat transparent pixels as black or white, but in the absence of that option I'd expect them to be treated as zero and argue that it's more useful than always generating a colored rectangle the shape of the layer.
I've also reverse-engineered (sometimes with the help of public-domain references) half of the major Photoshop blend modes and incorporated them into compositing software so we could import Photoshop files.  Mimicking what Photoshop does is different from doing it right in many cases.
And yeah, premultiplication causes real problems for people who don't know what they're dealing with when compositing.  Pre-multiplying was just a bad idea that unfortunately we have to live with.  Kinda like interlacing.
Thanks for the reply.