I thought it would be fun to look at how to paint folds and wrinkles in today's "how to do it" post.
The key thing is that light reflects in basically three ways when cloth has a wrinkle or fold. There's the "normal" or unwrinkled portion (labeled 2). Then there's the peak of the wrinkle, which will reflect light directly to your eye and therefore be the lightest (1). And lastly is the shadowed region (3).
What I've done is put a yellow dot on each of these areas so you can see where in a particular fold they're showing up. And then I color sampled that area, the results of which are shown below the photo. These are screenshots of the color analysis. The points are shown within their color rectangle (the horizontal axis goes from least to most saturated and the vertical axis goes from darkest to lightest). That information can be gotten from the color rectangle itself but is also shown at the bottom as a slider. The top slider is hue (color), the middle is saturation and the bottom is darkness (or tone).
The key point I wanted to make is how NOT large the differences are, especially between 1 and 2 (the light regions). But even the shadowed area that looks so black in the photo isn't really THAT much darker than the others.
Another way to see it are the rectangles I drew at the right. They're simply larger color samples of those three target points. Notice how, by themselves, they look like "pretty dark" and "not quite as dark". Yet on the actual photo they come across as quite different indeed.
The takeaway is that in painting, you don't need to be too extreme and, in fact, you shouldn't be if your aim is something that looks real. It doesn't take a huge difference in tone for our eyes to see quite a bit of variation.