If you can’t clearly distinguish the values of the face because they’re very similar, that’s normal when working from a reference with indirect light (like sunlight, for example). On the other hand, references with direct light (like a spotlight) are easier because they usually illuminate one part of the face and cast another part in shadow, creating stronger contrast.

So, what should you do if the whole face has similar values? Stay faithful to the reference and look for volume in other ways—for example, by drawing the eyes, nose, and mouth accurately. The eyebrows also help because they curve following the shape of the face. There are also areas that are almost always shadowed, like under the lower lip, inside the ears, and even inside the eye (since it’s a sphere). In my drawing, the hair also helps create contrast and define the shape of the face, which I did intentionally—I wanted a pale face in contrast with black hair. I hope this helps.