Wi-Fi will work on any wi-fi network, as long as the signal is unencrypted (from what I understand). However, there is nothing you can do on your private Wi-Fi that you can't do on the AT&T 3G signal. You can't browse the web, etc., and you can't transfer books from your computer. It's only for buying books from the B&N store and getting your subscriptions. The B&N Wi-fi network lets you browse through whole ebooks, but that is only in their stores. The only reason you'd need to connect to a private wi-fi is if you were out of AT&T coverage (e.g. out of the country on vacation).
And yes, e-ink is only in black and white at the moment on any device. A color e-ink prototype was announced today, coincidentally enough, but it's still a few years off, I think. One of the selling points of the nook is that it has color at all: no commercially available e-ink reader has any color on it, and all navigation is done on the e-ink screen, which has slow refresh time. The nook's color touchscreen is supposed to improve navigation while keeping the e-ink advantages for reading the actual text.