Not from Amazon unless you root the NC or run a different OS that allows you to install the Amazon Kindle book app. You might be able to sideload a Google epub book, but I don't know for sure. Maybe someone else can chime in. Oh, and there are some publishers (I believe TOR is one) that don't have the DRM security so you should be able to sideload books from them.
if you activated you nook color with Adobe Digital Editions, you can download Google play books and Kobo books and sideload them onto you're nook color with Adobe Digital Editions.
Unfortunately, as @petesnook mentioned, you can't put Amazon ebooks onto your nook color without doing some fiddling, either with the device itself or the ebook.