I created the piece of artwork above by drawing it first in pencil, then ink, then scanning it and adding texture and color in Photoshop. Man, I love Photoshop. Liz actually taught me this great way to color black and white artwork digitally using Photoshop and here’s how you do it:
1) Make a black-on-white line drawing with whatever tools you like, scan it in, and open it up in Photoshop.
2) Change mode to Grayscale. (Image menu > Mode > Grayscale)
3) Go to the Window Menu and select the Channels window so you can see it. Make a copy of the “gray” (ie, the only) channel by selecting “Duplicate Channel” from the drop-down menu. Rename the new channel, for instance, “b/w art”.
4) Change mode to RGB. (Image menu > Mode > RGB) If you get a message asking you if you want to merge the layers, say no.
5) In the Channels Window, turn off all of the channels except for your new one, “b/w art.” Command-click (Mac) or Control-Click(Windows) on the icon for “b/w art”. This will select the drawing in a way that preserves shading (unlike using the Magic Wand).
6) Go to the Window Menu and view the Layers window if it is not open already. Make a new layer by selecting “New Layer” from the drop-down menu. You can call this layer “color art” or something like that.
7) Invert the selection by going to Select menu > Inverse. (UPDATE: Disregard this step from my original post.)
8) Go to Edit menu > Fill and fill the selection with black. Now you can deselect.
9) Lock the transparent pixels for that layer.
10) Turn off or discard the layer for the original drawing. You can make yourself a new background by creating a new layer (Layers menu > New Layer), dragging it below the “color art” layer, and filling it with white or whatever color you like.
11) Now back to the color art layer you created: you can color it with whatever tools you like (paintbrush, gradient, select/fill, etc.). You can set the layer to be opaque or transparent, and so on. Coloring will *only* color the line parts, and any original shading, etc, will be preserved. Ta-da!
***ADVANCED MOVE: This works if you have a layered file as well. Change mode to grayscale. Turn off all layers except the one you want to create a separate channel for and then duplicate the gray channel. Do that for each layer in turn. Once you have a separate channel for each layer, convert back to RGB. Then follow steps 5-11 for each layer.