I adjust my Brush Tool to about a 15% Opacity, and if you look at the image on the left, I've circeld in red the areas where i'll slowly spot brush highlights of varying brightness. It's really up to the colorist how he or she is interpreting the color of the arm and how light is affecting the skin.

I do some more extra 'reflective light bounce' on the tricep area. With this, I used the Pen Tool to trace a specific shape of the light I want. I transformed the path into a marquee selection with a feathering of about 4 pixels, a bit sharper from the one I used for the bicep area.

Airbrushed in Screen mode, at about 20% Opacity.