Last summer I installed Android 4.0 (Ice Cream Sandwich - ICS) on my Nook Color. I used a technique described on the XDS Developers forum that let the Nook boot from a microSD card. The developer (verygreen) of this technique has apparently moved on to new challenges.
In the meanwhile, Android 4.2.2 (Jelly Bean) has dropped and there are CyanogenMod ROMs for the Nook Color using the microSD card technique.
The thread above has continued to get comments and questions and leapinlar has been very helpful in fielding these.
So here's how to install CyanogenMod 10 (CM10) Jelly Bean on a microSD for a Nook Color.
Use a Sandisk Class 4 8GB microSD card as recommended here. Use a microSD USB adapter and insert the microSD and USB adapter into your computer.
leapinlar has updated verygreen's microSD image to support ICS and Jelly Bean. Download the latest generic-sdcard-v1.3-CM7-9-10-larger-Rev[?].zip from here. Unzip and extract generic-sdcard-v1.3-CM7-9-10-larger-Rev[?].img.
If you're running Windows, download win32diskimager-RELEASE-0.3-r27-binary.zip from here. Unzip and extract the folder win32diskimager-RELEASE-0.3-r27-binary. Run Win32DiskImager.exe and write leapinlar's image (generic-sdcard-v1.3-ICS-large-Rev[?].img) to the microSD card.
After Win32DiskImager is done with writing, eject the microSD and USB adapter and re-insert it into your computer.
Download a CM10 Release Candidate build from here. Pick the newest from the top of the page. If you're brave download a CM10 nightly build from here.
Download the Google apps for Jelly Bean (currently gapps/gapps-jb-20130301-signed.zip) from here.
With the newly imaged microSD and USB adapter back in your computer, copy the CM10 ROM zip and Google apps zip into the root partition (the only one you can see) of the microSD card. Don't unzip these files.
Eject the microSD and USB adapter from your computer.
Make sure that the Nook Color is powered down and insert the new microSD card into the Nook. Power it up.
If the Nook doesn't boot at this point, revisit the microSD and USB adapter and the downloaded .img file. Explore alternatives perhaps using a different microSD and/or USB adapter and redownloading the .img file.
Sit back and watch. It will take a few minutes and lots of messages will scroll by. Then the screen will go black. Power up the Nook.
I still had to use the Google Play Store to install Gmail and youtube.
It's probably still worth reading my Ice Cream Sandwich errata.
The staying power of the Nook Color is incredible. It will soon be 3 years old and is running the absolute latest Android operating system.