How to paint skins of line art characters
Here, to show you typical ways to color line arts, I'll give some examples of painting skins of characters.

1. Use Painter's watercolor brush

I open line art file with Painter and simply paint light orange on character's skin with Watercolor brush. Following the correction of excessively painted area with Watercolor Eraser, the highlight areas are also lighten carefully with Watercolor Eraser, of which opacity was set to less than 10%. At this point, I fix the color by using Dry command in Canvas menu. Then the shadow areas are painted again with darker color by using low-opacity Watercolor brush. You can get better gradation and rich shadow by painting and drying thin (low-opacity) color repeatedly rather than painting all at once with dark color. Because feeling of coloring with Painter's watercolor brush is much alike as coloring with real watercolor or color ink, I think this method is easy to get used for a beginner of coloring with PC. You can get realistic touch of paper surface by using Painter's watercolor brush, but sometime it becomes a weak point of this method because of difficulty to correct colored area lately without leaving traces.


2. Use Photoshop's layer and airbrush

First, in this method, the line art must be transferred to a new transparent layer, which is set on a background layer. To transfer line art, I cut the whole line art from the background layer and paste it in a new alpha channel. A new layer is made on the background layer and a selection area is made from the alpha channel. Finally, the line art reappear on the new layer by filling the selection with dark brown. I paint basic color of skin (flesh orange) on background layer, then the shadows are added using airbrush tool on a new layer set between the background and the line art layer. Adding shadows on the independent layer makes it easy to correct mistakes later. The highlights are also added on a new layer as same as shadows. Because the line art layer is always on top of layers, you can preserve and see the clear lines of character while working.


3. Use Painter's brushes

I put basic color (flesh orange) with Painter's watercolor brush followed by drying canvas. Then shadow areas are added with watercolor brush and the canvas is dried again. After putting highlights with paint brush (usually, I use customized "Oil Pastel" brush), I make borders of colors smooth with customized "Captured Bristle" brush. This is my favorite method to paint with Painter because of likeness to painting with acrylic color. It is also easy to correct details later.

<- This sample shows steps to paint in method 3.
Of course there are lots of way to paint character's skin besides the three ways show above. I think it's always good start point for beginners to know and try to copy the methods your favorite artists are using.