Open your gray rendered BMP in Photoshop, save it as (filename).PSD.

Open your first mask render, I typically start with the object furthest in the background and work my way out to the foremost object.

Using your magic wand tool, select the white area (see selection footnote), go to Select on the menu and choose “Save Selection”.

Switch to your gray-render (do not close the mask render yet), go to Select menu and choose “Load Selection”.  If the name of the mask isn’t in the first box, open and select it. Click OK. Once the area is selected over your gray-render, Hit CRTL-C and then CTRL-V(CMD key for MACS). (Copy & Paste) Now your grayscale will no longer have the selection dots, and you’ll have a new layer. Rename this layer appropriately.(double click on the name, type it, hit ENTER)

Now you can close your mask render. Repeat this process for the remaining Mask Layers.  Don’t forget to save your PSD file after each addition.

NOTE: Remember when a selection is made, switch to your "Grey Layer" to copy from, then switch back to a new layer or the proceeding one. (hitting CTRL-V will paste a new layer if none is created).

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Ockham's razor- It's that simple