it is called subsurface scattering. its for the same reason your finger turns bight red if you shine a light ight behind it. When the light goes into the skin it bounces all over and produces a saturated red or orange glow. in the case of the terminator of the shade, that is where the most subsurface scattering happens because of the angle that the light hits the form.

apples, 5 min each

Check your perspective, that's what is off the most.

Nice pieces. You have a good understanding of color and value relationships but you need to work on your brushwork and 3d form. I'm specifically pointing out the overuse of the airbrush tool, if you overuse it, your images can become lost and the shapes don't read well. This would also include relying on "stamp" brushes instead of creating your own shapes and forms. To improve your form and brush work I would recommend doing a mastercopies in either landscape oil painters or digital painters. And try to do the study only using the default hard round brush, your be surprised how much you can learn and get quality images with such a simple brush. A great example of an artist who does this is Devin Korwin. Try focusing on clean shapes and better depictions of 3d planes. Overall, I think you have a lot of skill, just practice placing clean planes to indicate accurate form and work on your brushwork. I kind of showed you what I meant in a paint-over of your campfire, it's rough, but it shows cleaner shapes which in the end will prove a more pleasing picture. Keep on practicing and have a great day.

Nice job on these. Illustration is all about composition. In the mountain landscape, you need to add some variety in your edges to show some atmospheric perspective. The one with the guy and the tree, the composition is too even. Your doing pretty well with the first portrait, but the second one is missing form because you focused on color and not values. I did some sketches to show you what I mean, I hope it helps :)