Tutorial Description - In this example, I've used a picture drawn in Adobe Illustrator. Whilst it is entirely possible to both draw and colour your artwork in this package, the technique being demonstrated here requires the use of Photoshop or an equivalent. If you would like to experiment with this file, the .ai version is available as a download at the bottom of the page.
First of all, select your entire line drawing, making sure that there are no fills, just the lines, and copy it. Take it into Photoshop, and open a new document in RGB mode with a white background. I always add 20 pixels or so to the height and width generated by the computer so the image isn't pushed right against the edges. Place the image in the centre of the new document and OK it. The line art should now be on one layer, whilst the white background is on another