If you have a Nook Color and don’t want to install Android 4.0 Ice Cream Sandwich over the stock ROM then you can install CM9 on an SD Card and run ICS from that. The instructions to install CyanogenMod 9 on an SD card is a very simple process, does not require root, and sows not effect the Nook Color stock ROM.
Install CM9 on the Nook Color using SD Card:
If you cannot connect to WiFi Go here to fix it:
http://www.android-advice.com/2012/wifi-not-working-running-cm9-on-nook-color-sd-card-fix/
After Installing CM9 to the SD Card you probably want the Google Market. To get the market and other Google Apps follow this article:
http://www.android-advice.com/2012/install-the-google-market-on-the-nook-color-via-gapps-for-cm9/