... unless the filters through which the B&W images were shot were not pure 255 Red, Green or Blue. Then the color derived from the B&W pasted into the channels will be incorrect.
The above process gives you the flexibility to prescribe the color of the filter through which color information will be derived from each B&W so you do not introduce problems into the composite that you have to undo with further adjustments. It also provides some insight into how the filtered B&W images are combined to get a full-color comp.
The above process is not complex, but go with what works for you. As with most things in Photoshop, there are several ways to accomplish the final result. This topic has been discussed a few times here, but I figured I'd share my experience with the process because you asked.
simply change the XYZ values of the RGB channels in CUSTOM RGB set up in photoshop and save as an icc profile.
if anyone knows the of the filter set likely to have been used at the time the values my be available.