GooglePlay is not available in the Nook apps store, so you have to root (aka "jailbreak") the device to get Google Play.
(1) You can buy a SD card or download an SD card image that can boot off the card to an Android system with GooglePlay. You can also download the tools to make this card yourself. Then you have GooglePlay on the card side and Nook apps on the Nook color internal system.
(2) You can root the Nook System and install google apps to get Google Play on the Nook Internal system.
(3) You can install an alternative system on the Nook Color (like the card but internal) that has GooglePlay included.