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Nook Color
Triple boot MiRage CM7.2 (or MIUI 2.3.9), ICS CM9 (from uSD) and Nook Color (on EMMC) [xda-developers.com]
CM7.2-RC1 Mirage [xda-developers.com]
How to install to SD [xda-developers.com]
See the note on only using Sandisk Class 4 microSD
CM9 Samiam Nightlies [xda-developers.com]

CM7: Gingerbread (GB) Android 2.3
CM9: Ice Cream Sandwich (ICS) Android 4.0
MiRage: one of the CM7 build (currently CM7.2-RC1)
MIUI: another build of Android that has both GB (CM7) and ICS (CM9) versions available (currently 2.3.9)

Currently, the CM7 is more stable and performs better in Nook Color. Nevertheless, I found the CM9 to be at least functional too. Both support wifi and bluetooth in Nook Color using current builds.

Guide to install dual boot CM7/CM9 on microSD (triple boot with NC still in eMMC):
1. Download the dualboot image file from the top link above (or from http://techerrata.com/browse/nook/racks) and extract the img from zip file in a computer.
2. Download WinDiskImager from https://wiki.kubuntu.org/Win32DiskImager
3. Put a 4GB or larger microSD into the reader on your computer and burn the image onto it using Win32DiskImager (all previous content will be erased)
4. (Optional) Expand the SDCard partition using Easeus Partition Manager if the card is larger than 4GB
5. Power off Nook Color and insert the card
6. Power on Nook Color and it will boot into CM7.
7. Go to setting and connect to wifi.
8. Power off Nook Color
9. Power on Nook Color while holding both vol+/vol- button until it enters alternate boot. It will start up CM9.
10. Go to setting and connect to wifi.
11. Power off Nook Color.
12. Power on Nook Color while holding the "n" button to enter boot menu.
13. Use vol+/vol- button to go up and down and "n" button to choose "SD Boot" and "Recovery(uRecImg/uRecRam)"
14. Go down and press "n" start the boot in recovery mode.
15. Using the same buttons, choose "Install zip from SDcard" > "Choose zip from SDcard"
16. Choose, gapps-GB-20110828-signed-DualbootSD.zip. Enter with "n" and then go down the list to start the installation.
17. Now install the gapps-ICS-20111125-2-DualbootSD.zip likewise
18. Now you are done with dual boot CM7/CM9 installation using 1/15/2012 builds. Just use the power button to go back up the menu and reboot. It will first boot into Nook Color. Power off and reboot it will enter CM7 by default. If you want to go to CM9, just power on with vol+/vol- button pressed.
19. To update the MiRage CM7 or ICS CM9, download the updates zip file from the dual boot thread directly on the Nook Color (do not unzip). (Alternately, you may install ES File Explorer from Play Store and access your computer share drive and copy to the SDCard folder). Follow step 12 to 15 above and pick the update files for installation. The proper way is to install the CM7 update followed by gapps-gb installation again for CM7 update. For CM9 update, install the ICS update followed by the gapps-ICS file again.
20. If you want to use MIUI GB or MIUI ICS in primary boot instead of CM7 MiRage, you will need to format the system1 and data1. You will loose all data by doing so. Once you've gone through the procedure above, you should be able to follow the instruction on the thread to install MIUI.

The procedure above have been tested using a generic 4GB Class 4 (CO4G Taiwan) and a Transcend 16GB Class 4 microSD cards.