Okay, so we've obtained a line drawing-- pencil, pen-&-ink, or (as in this example) something in between. It's a single-layer picture: all of the information is right in the "background" layer, as Photoshop calls it. If I were to start painting directly onto it, even with a semi-transparent tool like the airbrush, eventually I'd cover it up. Indeed, almost immediately, the paler grayscale values in her hair, shadowlines, etc., start to disappear.
And there she goes, fading away under a green mist... What to do?
In Photoshop, we can copy the line art onto a mostly transparent layer. That way, we can add the color behind the line art layer, so we can't cover it up accidentally.
In other words, instead of opaque black, opaque gray, and opaque white pixels, we'll end up with opaque black pixels, semi-transparent gray pixels, and completely transparent pixels-- what I refer to as "line art on a mostly transparent layer."
The following is a brief synopsis of the method I use. If you're familiar with Photoshop jargon, you can just print out this list and be done with this tutorial. Or, if you prefer, follow along as I demonstrate each step.
- Step 0 - Get your Line Art
Get some line art into a plain old flat, single-layer, background-only Photoshop image.
- Step 1 - Make a Temporary Channel Mask of your Line Art
Switch to the Channels palette, pick one channel, and Duplicate it, with the Invert box checked. Display the ordinary channel(s) again (either click on RGB, or click on Black, if your image is still grayscale.) Switch back to the Layers palette.
- Step 2 - Add a Transparent Layer
From the Layers palette, add a new layer. Make sure that it's the currently active layer.
- Step 3 - Copy the Line Art in
Under the Select pulldown menu, choose Load Selection... Load from the temporary channel mask you made in Step 1. Click the OK button. Photoshop's marching ants appear. Make sure you're painting with black. From under the Edit pulldown menu, choose Fill... Use the Foreground Color, at 100% opacity. Under the Select pulldown menu, select None.
- Step 4 - Clean Up
Switch to the Channels palette and delete the temporary channel mask you made in Step 1. Switch back to the Layers palette. Empty out the background layer, since the info's in your line art layer now.
- Step 5 - Start the Fun!
Add a new color layer behind the line art, save your new .PSD file, and start coloring!