Here's how I would achieve that effect:
1) Create a BW Effects layer, and select the mode that gives the most pleasing effect.
2) Edit>Copy Merged, then paste that into a new alpha channel
3) Invert the alpha channel
4) Delete or disable the bw effects layer
5) Select the active pixels (Control key (PC) or the Command key (Mac) while you single click on the alpha channel in the channels panel)
6) Create a new solid color layer using the purple you used above
The active selection of the alpha channel will automatically be applied as a transparency mask. That SHOULD do it. Good luck.