So which version of Android do you normally install when you hack it?
Loaded question there.
Hack #1 - simply root it and then sideload the market and other apps to it. The Nook Color runs eclair (2.1).
Hack #2 - NookieFroyo. 3 ways to do it... first and easiest way is to dd the image to a microsd card and run directly off the microsd not touching the stock firmware. Second is to flash it to eMMC (embedded flash) and run it as your stock OS. 3rd way is to flash a dual boot bootloader and run both stock and froyo side by side. Personally, option #1 is the easiest (provided you get yourself a decently fast microsd card).
Hack #3 - Honeycomb preview image. Again, runnable but slow off microsd, or flashable to eMMC, or dual bootable as per #2.
Hack #4 - CyanogenMod 7 nightly builds. Currently requires you to flash to eMMC, but is possible to set up under dual boot as well. CM7 is a work in progress and is buggy, but usable. There is currently no way to set it up on a sd card.