Welcome to part one of the two part Digital Coloring for coloring comics Series. In this tutorial we will explore how to prepare line art for coloring comics in Adobe Photoshop. If you require line art to practice with you can get it here. This tutorial(s) makes use of the channels palette extensively, so if you are unfimiliar or uncomfortable with them you can use layers with some modifications, but it is recommended that you knuckle down and get into using channels as they are key to using Adobe Photoshop to its full potential.
Since we recieved the line art as is (on purpose) we are just going to do a quick fix for our line art for illustration purposes in this tutorial (step 1). It is recommended however that you follow the 'Scanning Line art Tutorial' for best results. Original Scan (below): (Line art by John Rauch Jr. www.pensnpixels.com)
1. Levels Adjustment.
<ctrl L> Brings up levels palette
Slide the arrow on the right towards the middle until the grey noise (pencil marks in this case) is gone. You may notice some of the black in the line will fade, so you need to slide the left most slider a little ways towards the middle. This will help darken the line again, but don't over do it, as you do not want to lose the integrity of the original line art. Note: You should remove any dark flecks or aberrations first as to get rid of them with levels will degrade the line art too much. You can do this buy either using the eraser (h) or by using the brush (b) tool with the foreground color set to white.
Before and After Levels Adjustments.
Before
After
2. Lets make an alpha channel for our Line Art.
Go to the channels palette and create an alpha channel. Click onthe little arrow beside the channels pallette and select New Channel, you must turn the color swatch to black, and make it 100%.
Now you can name it whatever you like although I suggest you name it Line Art, as it will become easier to identify if and when you get many channels going. Select ok.
With the new line art channel selected, fill it with white Alt+backspace (with foreground color selected as white). Your Line art channel should look something like mine to the left.
3. Now we need to make a selection of the Line
While pressing the <ctrl> key, click on the little picture window beside the blue channel. This will load it as a selection. You should now have a marquee on white. Now <ctrl + shift+ I> inverse the selection.
4. Fill the selection with black.
Make sure your foreground color is black and then hit <alt+Backspace>. As demonstrated in the figure to the left.
To the left is what your channels palette should look like at this time.
6. Remove the original Line art
Okay now that we have the Line art stored in an Alpha Channel we can remove the original line art from the RGB channel. This is simple. Select the RGB channel - fill the channel with white (make sure the foreground color is white) and then press alt+backspace. Now you should no longer have anything in the channels R,G,B or RGB.
7. Make the line art visible.
More Tutorials: 
