It should not affect your ability to purchase, download and read books. I can still do all that with my first edition Nook.
I'll second what bklvr said.
You should still be able to buy, download, read books on Nook Color for a long time.
I recently read where NC is being phased out or discontinued. I am presuming that means no more updates and eventually (if not now) the inability to purchase accessories - cover, charger, etc.
Chargers and cables are already getting a bit scarce. You can still find them online with B&N on a good day, but of course, you'll pay their premium price. I'd pick up a spare set just in case. You can charge without them, but more slowly. The proprietary cable is the more critical piece.
If you want a custom-fit case or skin, I'd buy one sooner than later, since 3rd party manufacturers are probably trying to get rid of any stock on hand.
Am I correct in presuming that you will still be able to purchase, download, and read on the NC? Or will that eventually not be possible as well?
You should be able to continue to purchase B&N books for some time. Subscriptions are probably safe, but there is a history of some magazines suddenly not being compatible with older devices (notably PopSci and Discover on the NST/NSTG).
The best protection against obsolescence is the fact that you can easily root the device, and load alternate firmware on it. Even if B&N abandon's it completely, you'll still be able to get some use out of it running alternate software.