Kindle uses an Amazon specific book format that no other store or device uses. This leads to lock-in with the Kindle and the eBooks purchased. There are tools of questionable legal use that remove the DRM and convert the book content to a format that may be usable by the nook.
You can google to find out more if you are interested. I'm not condoning this particular site, but is an example of the type of tools that exist.
Another option is to root your nookColor. This would allow you to install the Kindle application on your nookColor.