What I want is for the image to turn all grays into transparencies of black, which doesn't seem to work when I simply use "Colour Range" to remove white from image.
-Using Adobe Photoshop.
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This method is totally non destructive and you can reset this back to what it was before.
You might also want to check out this link if you're looking to create those Shephard Fairey/stencil style effects.
Make a new layer. Flood fill with black. Attach a layer mask to the layer.
Select the greyscale layer to give it focus, then "Select All"
Switch to the channels palette, select the layer mask for the black layer, make it visible and then paste. This should paste the greyscale image into the layer mask of the black layer. Make the layer mask layer "not visible."
Switch back to the layers palette, hide the original greyscale image.
White will be opaque in the layer mask. Invert the colors of the mask if you want the blacks in the original greyscale to be opaque.
You can add a color-overlay layer style to alter the color of the black layer.
In the image below, "Layer 0" is the original greyscale; "layer 2" is the new black layer