Ok, so a user on the forum asked me if there was something wrong with the technique he's using in coloring his character. Now I'm a visual guy, so rather than just tell him what I think is right or wrong with what he's done, I decided to just show how I would go about doing it and hope that people would get something from it. So here we go.

Left is an arm I drew in pencil with which I'll be working with. I've labeled the different muscle groups because in coloring an arm, its important to know your muscles.

I've already layed out the flats, and on the right side you can see the three different shades I'm using on the skin tone.

My way of coloring a part of the body like an arm or a leg is pretty basic. I start coloring it as a whole and then go inward with adding details.

 

Like drawing, I see the arm shaped as a cylinder first, so I start adding shadows using the middle shade color.

Using a soft airbrush in the Brush Tool in Photoshop in Mutiply Mode at about 30% Opacity level, and just follow general shape of the arm.

I'm working on the premise that a main light source is located in front, with the central focus on the face/ neck area, the outer radius of the light hitting the arm along with other parts. This setup is based on my interpretation of what I saw in the image posted in the forum.