Here's a tutorial from Kiara demonstrating a good use for the Layer Unify tool! Thanks Kiara!

--Z, Webmistress
(Note: This is a fanart colored piece of this picture. Visit the original artist here.)
(Click the thumbnails to enlarge the pictures)

First, On layer 0. make a sketch of your picture. I made the sketch in black watercolor with 240 opacity. When you have a sketch your satisfied with, try to clean it up a little to make lineart easier.

Next go to layer 1 to do the final lineart. I did this lineart in black watercolor with 250 opacity to simulate a pencil. Use the bezie tool (click twice on freeline) to do long, smooth curves. When you have traced your entire picture, go back to layer 0 and select the white rect tool (click twice on white) and select the entire picture. This erases the sketch underneath the lineart.

Staying on layer0, pick a color for the skin. Using the solid tool, lay down one solid color for the skin. If you went anywhere outside the skin area, use the white tool as an eraser to clean it up.

Still on layer 0, pick a darker color for shading. Lay down solid flats for shading in desired areas, keeping in mind the direction the light source is coming from. Then, select the shade off tool (click twice on tone) and set it to 240 opacity to blend the colors smoothly. Add highlights and darkness where you think are needed. When you're COMPLETELY done with the skin areas, go to layer 1 and select the layer unify tool (click twice on copy) then select the entire image. This brings everything on layer 0 up to layer 1.

Go to layer 0 to do the next color, using the same process for each color. After you are done with a color, shaded and everything, go to layer 1 and use layer unify tool again. Do this after EACH color to make things easier. Follow this process until the picture is completely colored and shaded.

On layer 0, add a background, and since the rest of the picture is on layer 1, the background won't disturb the picture. Use layer unify one more time on layer 1, to get the whole picture all together on the same layer. Here is the finished picture.