As Joseph suggests, you can use the picture control to do this - if you're shooting raw. If you shoot jpeg, the jpeg is permanently converted to B&W, using whatever parameters are set in the camera, and there's no undoing it. If you shoot raw, you'll get a B&W display, as you desire. The raw file will still contain the raw, unprocessed data - including all of the color information. It will also contain a jpeg "preview" image - which will be B&W, per the jpeg discussion above. But since it is in fact a raw file, any of the raw converters will be happy to process the data into a full color image for you, if you so desire. Lightroom in particular does a rather odd dance. If you shoot this way - and I do, quite frequently - you see a bunch of B&W images as the card is imported into LR. Then as Lightroom renders the preview images as it does as soon as the import is complete, you can watch the entire shoot "turn" from B&W into color!
I shoot quite a few images that are intended to be presented as B&W, so I shoot raw with picture control set to monochrome quite frequently. I find this to be very helpful in evaluating B&W composition, even though the colors may not convert to B&W the same way that I'll do it later.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!