This assumes that your B&W render is a mask render, not just a desaturated version of your full pic. Open the B&W PSD file, in another layer paste your color render of the ship. Your ship should show as white in the B&W render. Use the magic wand and select the white. Next switch the selected layer to be your color render and go edit>copy. The result will be a copy of just the ship. You can paste this over your backdrop in either a seperate PSD doc or as another layer in the doc you copied it from.