All three of the above previews will be quirky.
I've been testing font sizes, margins and most importantly for me, images. (Using MS Word, MS Works with file types of .doc and .rtf) The books I'm finalizing include heavy illustrations and images.
Note: If you are using strictly text then you are in the best of luck. Most of my comments below will not apply to you. Other than forcing page breaks in your document, you will have the least trouble.
Once you start to embed images, or have heavy illustrations among your text, then you will pull a few hairs out getting everything justified. (I'm excluding the cover image - it is a separate file.)
If color is paramount, then the Nook Color preview is important to you. However, please remember that the viewing screen for the B&W Nook has been different in my testing. The Nook Color appears to offer a longer viewing area. Your properly formatted Nook Color book will get cut short in the B&W Nook. The B&W Nook will take your embedded images and convert them to B&W.
The Nook for the PC has a two page, open book layout. You'll find that once it looks good in Nook Color, the PC version may still have issues.
Justifications for embedded images are very unpredictable and sometimes just don't work. If you would like a centered illustration nicely surrounded by typed words I wish you the best of luck. Sometimes I got my wish, sometimes I didn't.
If you find that your Book is comprised of images and illustrations more than text, then I would combine the two and place a full page image on your Word document page. (With a page break between.) I think this is the route I'll end up going for a couple of my books.
Synopsis. (For those who have heavily imaged books, and are still reading this.) :-)
1) Create a full page image, (600 px X 750, or 1200 px X 1500 px) px = Pixels
(Remember, you should be able to use every pixel available on the top and sides.) Let the image bleed off.
2) Place your image(s) and text and save as a .jpg with the maximum quality.
3) Center the image on a single page of your Word document.
4) Force a new page by inserting a page break.
5) If you need a TOC before, you should be okay inserting this on its own page with a page break after.
6) If you have issues, it will most likely be that your images are getting cut off at the bottom. In some instances, you may only get to utilize 3/4 of your image length.
Lastly, if you find anything related to the above and would like to add it, please reply. Eventually we will get the correct combination and will know exactly what to do.