The first i'll do is use my Marquee Tool again, feathered to about 5 pixels, and select areas at the right side of the arm where the radius of the light is hitting it, to apply what I call a 'reflective bounce' of light to the skin. What this does is add more depth to the arm as a whole and let's you see the roundness of the biceps and deltoid area.

 

Take your Brush Tool in Airbrush mode still, at around 40% Opacity, and instead of putting it on Multiply, place it on Screen Mode and start shading in the highlights.

What I got is the highlights that are circled in red. I made the bottom area of the bicep is much brighter than the top part again because of how the light source is positioned.

I'd like to note that all throughout this coloring process, I've only used the second shade of my color scheme, from Multiply Mode to Screen Mode.