If you want to digitally colour your line work, there are loads of ways of to do this. Adjust the image mode to colour: -Image -Mode -RGB. Then in the layers tab drag the background layer onto the 'new layer' icon, what'll create a copy of the background onto a clean and adjustable new layer (otherwise you wont be able to move the outlines onto the top in the layers tab (see Fig 6.).

Fig 6.

Delete the bottom 'background' layer and rename the 'copy of background' layer to "outline". Next check the preserve transparency box for this layer. Now from here there are two good ways to progress...
Method one: While having the outline layer selected, go to the layer drop down menu and select "multiply".

Fig 7.

After that create a new layer and call it BG or Background. Now on the background, you'll be able to colour underneath the lines of the outline layer, but on top of the white of the outline layer! (see Fig 7.).
Method two: This is what I do, since there are a few small advantages you can use later on.. Go to the channels tab and click the circular channel selection icon, then the image should be filled with dotted lines (see Fig 8.).

Fig 8.

Next click delete - this will get rid of all white areas. Unselect the channel selection, and then move back onto the layers tab. Since the preserve transparency box is check you can then, re-apply black to the outlines by using the pencil tool with a brush size of 300+, then going over the whole picture to restore black to the outlines. Create a new layer underneath called BG or background and you'll be able to colour underneath the remaining outlines! (see Fig 9.).

Fig 9.

Summary:

• Change image mode to RGB
• Turn the background layer into an adjustable outline layer
• 'Multiply the layer, then colour underneath
• OR Delete the while area of the outline layer buy using the channel selection icon
• Then re-colour the black lines with the pencil tool while preserve transparency is checked