Hey Chiara,
Thanks for the compliments and I'm delighted to answer any question you may have.
Let's talk about these two concepts first: bounce light and subsurface scattering. The former is almost essential to rendering, whereas the latter is merely a cool trick, a nice-to-have but entirely optional.
There are plenty of videos to understand how bounce-light works, and its close relationship/antagonism with ambient occlusion. I recommend Marco Bucci's youtube videos on colors, light, and shadow as the best illustrated videos on the subject.
When it comes to applying it in my painting, I have a pretty algorithmic method. First step is to establish shadow and light on a form, and knowing which parts are in the shaded from the main light source. Bounce light is pretty much only visible within a shadow, and affects lit areas of a volume SIGNIFICANTLY less. Second step is asking myself if any part of that shadow shape is "exposed" to the ambient surroundings, i.e. is it away from any crease, folds, or occluded part of a volume. For example, the left part of the forearms above is shaded, but very exposed to the green surroundings. The last step is including a soft gradient of the ambient color inside the shadow shape, while making sure that the core-shadow is still present and of a darker value. Experiment, play around, and have fun with it!
Sub-surface scattering (SSS) is fairly easy to do as it involves less thinking. It appears only in one condition - semi-translucent objects lit using strong light, such as in the case of an ear being lit from behind (see my profile pic!).
Skin, translucent/glassy materials, some cloths, and some organic materials benefit from SSS. Don't apply it to rocks, wood, or anything opaque. The easiest way to account for SSS is to consider that any point on a volume where there is a hard transition from light to shadow should have a band of saturated color on the edge of the shadow shape - that's light bleeding through inside the material into the shadow shape.
In some conditions, such as skin being lit from behind, you can use extremely saturated reds and oranges to emulate the blood and capillaries inside the skin. That takes a bit of practice, but typically the thin parts light up and the thick parts don't. Edges are also worth considering too. Use a photo-reference if available, and go wild with the saturation in your colors!
On another note, here's a photo study of some e-girl I found on Pinterest. I have a soft spot for that aesthetic 
Once again, the mixer brush was absolutely crucial to blending my colors easily and quickly.
Cheers all!
