Put my phone's SD card into a Nook Color - will it work?

It's not a good idea to swap a card between devices. Each one will try to write to it.

You could plug both into your computer and copy from one to the other or one at a time to your computer, then to your Nook. I did a phone to phone transfer when I got my new phone, which doesn't have a sdcard.

If you're borrowing a NC, I'd pick up a sdcard or use a spare and copy whatever you want to that.