Background Painting Tutorial in Photoshop by - Lisa Rye
Editing Lineart
How to make a transparent lineart:
To start with copy and paste your lineart onto a new layer. If you have any other layers make them invisible. Pull up the channels window.
Click on the 'load channel as selection button', then hit the 'delete' button. This will remove the white areas.
You'll notice that the lineart looks very light in colour after deleting the white. In order to make the lines black again, first hit the 'lock transparent pixels'. Basically this means that you can only paint over areas that are opaque. As your lines are the only opaque areas, this means you can change their colours without affecting the rest of the picture.
Select a big, thick, black brush and brush over all the area lines cover.
You might want to experiment with using different colours to get the idea of how the 'lock' opacity function works.
PS: Tutorial written with the permission of the Author! Lisa Rye