Now let's split the arm into muscle groups.

 

As you can see on the right image, the red lines indicates how i'm going to mask the different muscle groups, so it's really essential when you are coloring an arm that you know where the muscles are located and how each of it is connected to one another.

This is what I meant about coloring from the outside and then going inwards for the details.

The red lines are the paths I'm making for each muscle group, using the Marquee Tool feathered to about 5 - 8 pixels.

Remember that I'llbe selecting each one at a time and applying the proper shade.

Using again my Brush Tool in Airbrush and Multiply mode, with a varying Opacity of 20 - 40%, I slowly color in the details of the muscles, getting the left side of the arm (in the tricep area) darker than the right side (bicep area) in association with the light source.

Now if you'd notice that the 'cuts' in the muscle groups are soft, thisis because of my own preference. How much less your Marquee Tool feathering is will determine how sharp or soft the cuts will be.

At this point of the coloring process, all i'm doing is adding the shadows and dark shades on the arm. I like to finish everything on thedark tones first before I go and color in the highlights, this is to have a definite shape of the whole arm.

 

At the forearm area where its circled red, I've made the shade more darker from the rest of the arm because that part is generally much darker in relation to the light source.

Ok, so the dark parts are finished. Now to add the highlights.