Lesson
3i) It's time to create a transparent line art layer! Open the channels window (Windows>Show Channels) and click on the Black channel. Load the channel as a selection by clicking on the 'Load channel as a selection' (I love Photoshop) button at the bottom of the Channels window [].
Now invert the selection (Ctrl+Shift+I). This will select your black line art.

3ii) Now open the layers window (Windows>Show layers) and create a new layer by clicking on the 'Add new layer' button []. Name this layer 'line art'.

3iii) Press 'd' to get the default colours (foreground: black, background: white) and in your new 'line art' layer, fill in the selection with the foreground colour (alt+backspace). Your 'line art' layer now has some line art in it!

3iv) Make this layer indestructible (sort of) by checking the 'Preserve transparency' checkbox in the layers window. You now can't draw on any transparent bits in this layer. It's also good to put it in Multiply mode using the pull-down menu in the layers window.

3v) Now change your image mode to RGB (Image>Mode>RGB; Don't flatten).

3vi) Select the Background layer (Ctrl-a) and delete the line art from it (Delete). Fill it with a fairly bright colour which you won't use much in colouring your image. This will help you later as you begin colouring (it shows any spots you've missed).

Finishing up:
- You have a transparent and protected line art layer in RGB mode
- You have a coloured background layer