I am quite sure of the exact moment I understood that I was doing it wrong. My skin tones, in my art specifically, were dull and too dark. I was sensing something was wrong but couldn't wrap my head around what was wrong... until I opened my Brom artbook to see if there was something I could learn from him. That was the moment it dawned on me! Brom did not use black or grey as shadows in skin tones. Especially if you look at some of his artwork that has delicate females. The skin is super light. I also noticed that as long as the light area of a face is really light the shadow doesn't need to be dark at all to read as shadows, as long as they are just a notch down on the value ladder.
When I think back I cannot believe how long it took me to get it right. I mean, there is tons of Old Masters Pieces that does this all the time. It is as if I skipped the week of painting class where they taught shadows... wait, that's it. I did not go to school at all. If only someone had told me. Wait, someone did... Donato told me... 15 years ago. If only I had listened. Now, you listen...
Do not use black or grey for your shadows (or perhaps even dark colours at all).
Think of it this way: Skin is a little moist. It reflects differently than cloth or fabric. Moist surfaces reflect more light and is lighter than the clothes the figure is wearing, since cloth absorbs more light and reflects less ( Well that is all the scientific explanation I can muster ). But it is obvious when you look at people in the street. The faces and hands are light pinkish blobs sticking out from darker colours.
Still, many artist, including myself, tend to paint skin way too dark. I think, for my part, it comes from doing comics and having trained for so many years in doing pencil or black and white illustration. I still see it as drawing process where I establish shadows and cast shadows to describe form. I should be thinking more about surfaces and blocks of elements, when painting.
Here are a bunch of good and bad examples:
Bad skin: Too dark and too much black.
Good skin tones: More color, lighter tones and no leaning against black as a shadow color.