This is the most re-requested tutorial that I teach in the digital Illustration class. While many of the processes are interesting to try, this process is one that nearly every illustrator will need to use at some point. The result is a line drawing with a transparent background. This allows the image to be colored like a cell from animation in addition it allows for the line itself to be colored.
Scan a drawing in grayscale mode and adjust levels as desired.
Separating the line from the background
Select All and Copy the image.
Create a new file. File New. The size of the image will be the size of the image you copied.
Open the Channels pallette. Create a new channel. The default name will be Alpha 1. Select the Alpha 1 channel and Paste.
Return to the Layers pallette and make a new layer. Title the layer “line.”
From the menu bar go to Select > Load Selection. Choose Alpha 1 and click the invert box.
At this point I lock the transparency on the line layer. This allows me to paint the line any color I would like with a basic paint brush (the red in the above image is an example of painting with a basic brush with the transparency locked).